Add missing null-terminator on file macro definition string

[urshanselmann]

Avoids potential buffer overflow in subsequent reads.

To reproduce in the unpatched version, try

#pragma file __FILE__

somewhere in source passed to MOJOSHADER_preprocess().

Stack-trace from AddressSanitizer:

Heap-buffer-overflow on address 0x50200000273b at pc 0x62c95b88a117 bp 0x7ffcefe56af0 sp 0x7ffcefe562b8
READ of size 12 at 0x50200000273b thread T0
  at 0x62c95b88a116 strlen
  at 0x62c95c37d6fc handle_pp_identifier (mojoshader_preprocessor.c:1547)
  at 0x62c95c378320 _preprocessor_nexttoken (mojoshader_preprocessor.c:2133)
  at 0x62c95c377ca4 preprocessor_nexttoken (mojoshader_preprocessor.c:2173)
  at 0x62c95c378a64 MOJOSHADER_preprocess (mojoshader_preprocessor.c:2268)
  ...
  at 0x62c95b872224 _start
Allocated by thread T0 here:
  at 0x62c95b912053 malloc
  at 0x62c95c3841da MOJOSHADER_internal_malloc (mojoshader_common.c:12)
  at 0x62c95c379529 Malloc (mojoshader_preprocessor.c:73)
  at 0x62c95c37e54e find_define (mojoshader_preprocessor.c:454)
  at 0x62c95c37d62b handle_pp_identifier (mojoshader_preprocessor.c:1541)
  at 0x62c95c378320 _preprocessor_nexttoken (mojoshader_preprocessor.c:2133)
  at 0x62c95c377ca4 preprocessor_nexttoken (mojoshader_preprocessor.c:2173)
  at 0x62c95c378a64 MOJOSHADER_preprocess (mojoshader_preprocessor.c:2268)
  ...
  at 0x62c95b872224 _start