Fixes a number of memory issues reported by AddressSanitizer.

[gmbeard]

This PR fixes a number of memory issues reported by AddressSanitizer.

Memory Leaks

ASan detects lots of leaks. They all look similar to the below...

Direct leak of 128 byte(s) in 4 object(s) allocated from:
    #0 0x2b58317fb6f0 in __interceptor_malloc ../../../../gcc-8.1.0/libsanitizer/asan/asan_malloc_linux.cc:86
    #1 0x2b58326eb506 in make_stack_item /vagrant/toml_parse.rl:731
    #2 0x2b58326ed9b1 in toml_parse /vagrant/toml_parse.rl:322
    #3 0x401beb in main /vagrant/main.c:120
    #4 0x32e301d9f3 in __libc_start_main (/lib64/libc.so.6+0x32e301d9f3)

Indirect leak of 64 byte(s) in 2 object(s) allocated from:
    #0 0x2b58317fb6f0 in __interceptor_malloc ../../../../gcc-8.1.0/libsanitizer/asan/asan_malloc_linux.cc:86
    #1 0x2b58326eb506 in make_stack_item /vagrant/toml_parse.rl:731
    #2 0x2b58326ee2f2 in toml_parse /vagrant/toml_parse.rl:413
    #3 0x401beb in main /vagrant/main.c:120
    #4 0x32e301d9f3 in __libc_start_main (/lib64/libc.so.6+0x32e301d9f3)

The fixes for these are...

1. Add free(context) to the POP_CONTEXT(x) macro. (toml_parse.rl)
2. After the %%write_exec section, loop over the remaining items in the context stack, calling POP_CONTEXT(...) on each. (toml_parse.rl)

Invalid Memory Access

The examples/example.toml file fails ASan, giving the following error...

AddressSanitizer:DEADLYSIGNAL
=================================================================
==6764==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x2b6d54e31272 bp 0x7fffc51990d0 sp 0x7fffc5198808 T0)
==6764==The signal is caused by a READ memory access.
==6764==Hint: address points to the zero page.
    #0 0x2b6d54e31271 in __sanitizer::internal_strlen(char const*) ../../../../gcc-8.1.0/libsanitizer/sanitizer_common/sanitizer_libc.cc:171
    #1 0x2b6d54d7b22c in printf_common ../../../../gcc-8.1.0/libsanitizer/sanitizer_common/sanitizer_common_interceptors_format.inc:544
    #2 0x2b6d54d7bb52 in __interceptor_vfprintf ../../../../gcc-8.1.0/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:1505
    #3 0x2b6d54d7bc16 in __interceptor_fprintf ../../../../gcc-8.1.0/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:1550
    #4 0x2b6d55d0353c in _toml_dump /vagrant/toml.c:108
    #5 0x2b6d55d035f3 in _toml_dump /vagrant/toml.c:111
    #6 0x2b6d55d03416 in _toml_dump /vagrant/toml.c:97
    #7 0x2b6d55d03270 in _toml_dump /vagrant/toml.c:81
    #8 0x2b6d55d03d34 in toml_dump /vagrant/toml.c:208
    #9 0x401dc3 in main /vagrant/main.c:141
    #10 0x32e301d9f3 in __libc_start_main (/lib64/libc.so.6+0x32e301d9f3)
    #11 0x401158  (/vagrant/build/main+0x401158)

It appears to be a problem with lists. Ensuring that, if there is no name present, setting item->node.name = NULL in the start_list action appears to fix this. (toml_parse.rl)

NOTE: I've only tested these fixes with the provided example .toml files in the project. However, it looks as though these fixes may go some way to addressing issues #18, ~~#19~~, #20, #22, #23, and #24.