_strdup(NULL) in SetLastSPASMProblem when coming from write_labels

[adriweb]

On the latest source as of this date, calling spasm with -L crashes with a segfault on: lpErr->lpszFileName = _strdup(curr_input_file);. That's on errors.cpp:326

I've tried tracing the problem a bit, and it looks like curr_input_file is freed and set to NULL in write_file (export.cpp:192-193), and nothing else after that. So, when/if SetLastSPASMProblem is called later, it crashes.

Backtrace if you're interested:

* thread #1: tid = 0x8c5e2f, 0x00007fff8cd82152 libsystem_c.dylib`strlen + 18, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x00007fff8cd82152 libsystem_c.dylib`strlen + 18
    frame #1: 0x00007fff8cddcb79 libsystem_c.dylib`strdup + 18
  * frame #2: 0x000000010000c6b4 spasm`SetLastSPASMProblem(dwErrorCode=261, fIsWarning=false, valist=0x00007fff5fbfef20) + 132 at errors.cpp:326
    frame #3: 0x000000010000ca7a spasm`SetLastSPASMError(dwErrorCode=261) + 362 at errors.cpp:382
    frame #4: 0x00000001000066e7 spasm`parse_single_num(expr=".end", value=0x00007fff5fbff2ec) + 3447 at parser.cpp:450
    frame #5: 0x0000000100004f9f spasm`parse_num_full(expr=".end", value=0x00007fff5fbff37c, depth=0) + 447 at parser.cpp:548
    frame #6: 0x0000000100004d9f spasm`parse_num(expr=" .end", value=0x00007fff5fbff37c) + 63 at parser.cpp:120
    frame #7: 0x0000000100015339 spasm`write_defines_callback(define=0x0000000100600280, label_list=0x00007fff5fbff5a0) + 73 at storage.cpp:109
    frame #8: 0x0000000100002129 spasm`hash_enum(ht=0x0000000100102d10, enum_callback=(spasm`write_defines_callback(define*, _list*) at storage.cpp:101), arg=0x00007fff5fbff5a0)(void*, void*), void*) + 681 at hash.cpp:64
    frame #9: 0x000000010001517f spasm`write_labels(filename="/Users/adriweb/Downloads/TI/z80_ez80/helloworld.lab") + 175 at storage.cpp:62
    frame #10: 0x000000010000b0d4 spasm`run_assembly() + 1524 at main.cpp:187
    frame #11: 0x000000010000ba9b spasm`main(argc=5, argv=0x00007fff5fbff8c0) + 2171 at main.cpp:436
    frame #12: 0x00007fff911155ad libdyld.dylib`start + 1