Fix uninitialized variable in FatFile::parsePathName

[earlephilhower]

In an error case, it is possible that fname->len is not initted in the FatFile::parsePathName function. That length variable is later used to determine if the name was not set or was too long in line 508, resulting in undefined behavior. Fix by clearing on entry.

Detected by valgrind run in the ESP8266 repo:

2021-11-03T08:00:01.6217614Z ==7070== Conditional jump or move depends on uninitialised value(s)
2021-11-03T08:00:01.6220064Z ==7070==    at 0x3150FD: FatFile::parsePathName(char const*, FatLfn_t*, char const**) (FatFileLFN.cpp:507)
2021-11-03T08:00:01.6222013Z ==7070==    by 0x3107A9: FatFile::open(FatFile*, char const*, unsigned char) (FatFile.cpp:448)
2021-11-03T08:00:01.6223860Z ==7070==    by 0x310638: FatFile::open(FatVolume*, char const*, unsigned char) (FatFile.cpp:422)
2021-11-03T08:00:01.6225630Z ==7070==    by 0x2F5187: FatVolume::exists(char const*) (FatVolume.h:86)
2021-11-03T08:00:01.6227292Z ==7070==    by 0x2F572A: sdfs::SDFSImpl::exists(char const*) (SDFS.h:85)
2021-11-03T08:00:01.6228874Z ==7070==    by 0x2D32F2: fs::FS::exists(char const*) (FS.cpp:379)
2021-11-03T08:00:01.6230462Z ==7070==    by 0x192CFD: sdfs_test::____C_A_T_C_H____T_E_S_T____110() (test_fs.inc:119)
2021-11-03T08:00:01.6232199Z ==7070==    by 0x24F4BC: Catch::FreeFunctionTestCase::invoke() const (catch.hpp:5874)
2021-11-03T08:00:01.6233964Z ==7070==    by 0x220F53: Catch::TestCase::invoke() const (catch.hpp:6779)
2021-11-03T08:00:01.6235739Z ==7070==    by 0x24A24F: Catch::RunContext::invokeActiveTestCase() (catch.hpp:5473)
2021-11-03T08:00:01.6238099Z ==7070==    by 0x249A8D: Catch::RunContext::runCurrentTest(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) (catch.hpp:5445)
2021-11-03T08:00:01.6240343Z ==7070==    by 0x246DCB: Catch::RunContext::runTest(Catch::TestCase const&) (catch.hpp:5284)
2021-11-03T08:00:01.6242064Z ==7070==  Uninitialised value was created by a stack allocation
2021-11-03T08:00:01.6244345Z ==7070==    at 0x310664: FatFile::open(FatFile*, char const*, unsigned char) (FatFile.cpp:425)
2021-11-03T08:00:01.6246169Z ==7070== 
2021-11-03T08:00:01.6251775Z ==7070== Conditional jump or move depends on uninitialised value(s)
2021-11-03T08:00:01.6254096Z ==7070==    at 0x31510D: FatFile::parsePathName(char const*, FatLfn_t*, char const**) (FatFileLFN.cpp:507)
2021-11-03T08:00:01.6256315Z ==7070==    by 0x3107A9: FatFile::open(FatFile*, char const*, unsigned char) (FatFile.cpp:448)
2021-11-03T08:00:01.6258461Z ==7070==    by 0x310638: FatFile::open(FatVolume*, char const*, unsigned char) (FatFile.cpp:422)
2021-11-03T08:00:01.6260848Z ==7070==    by 0x2F5187: FatVolume::exists(char const*) (FatVolume.h:86)
2021-11-03T08:00:01.6262891Z ==7070==    by 0x2F572A: sdfs::SDFSImpl::exists(char const*) (SDFS.h:85)
2021-11-03T08:00:01.6264840Z ==7070==    by 0x2D32F2: fs::FS::exists(char const*) (FS.cpp:379)
2021-11-03T08:00:01.6266761Z ==7070==    by 0x192CFD: sdfs_test::____C_A_T_C_H____T_E_S_T____110() (test_fs.inc:119)
2021-11-03T08:00:01.6268827Z ==7070==    by 0x24F4BC: Catch::FreeFunctionTestCase::invoke() const (catch.hpp:5874)
2021-11-03T08:00:01.6270938Z ==7070==    by 0x220F53: Catch::TestCase::invoke() const (catch.hpp:6779)
2021-11-03T08:00:01.6273049Z ==7070==    by 0x24A24F: Catch::RunContext::invokeActiveTestCase() (catch.hpp:5473)
2021-11-03T08:00:01.6275817Z ==7070==    by 0x249A8D: Catch::RunContext::runCurrentTest(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&) (catch.hpp:5445)
2021-11-03T08:00:01.6277717Z ==7070==    by 0x246DCB: Catch::RunContext::runTest(Catch::TestCase const&) (catch.hpp:5284)
2021-11-03T08:00:01.6279120Z ==7070==  Uninitialised value was created by a stack allocation
2021-11-03T08:00:01.6280534Z ==7070==    at 0x310664: FatFile::open(FatFile*, char const*, unsigned char) (FatFile.cpp:425)
2021-11-03T08:00:01.6281714Z ==7070== 

[greiman]

I will soon add this change to several others for SdFat V 2.1.2.