[libfreerdp] Pass a zero-terminated string to freerdp_assistance_parse_file_buffer
```
READ of size 2 at 0x602000000091 thread T0
SCARINESS: 14 (2-byte-read-heap-buffer-overflow)
#0 0x4c6fb9 in StrstrCheck(void*, char*, char const*, char const*) /src/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:580:5
#1 0x4c6df1 in strstr /src/llvm-project/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:597:5
#2 0x56c9ba in freerdp_assistance_parse_file_buffer /src/FreeRDP/libfreerdp/common/assistance.c:743:6
#3 0x56b58e in parse_file_buffer /src/FreeRDP/libfreerdp/common/test/TestFuzzCommonAssistanceParseFileBuffer.c:11:11
#4 0x56b58e in LLVMFuzzerTestOneInput /src/FreeRDP/libfreerdp/common/test/TestFuzzCommonAssistanceParseFileBuffer.c:20:2
#5 0x43f5e3 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:611:15
#6 0x440994 in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__Fuzzer::vector<fuzzer::SizedFile, std::__Fuzzer::allocator<fuzzer::SizedFile> >&) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:804:3
```
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