[libFuzzer] Fix fd check in DupAndCloseStderr.

[charco]

Summary

This commit fixes the check in the return value from the DuplicateFile
function, which returns a new file descriptor. DuplicateFile can
return 0 if that file descriptor is available (for example, if stdin has
already been closed).

In particular, this could cause a bug with the -close_fd_mask flag in
some platforms: just call the fuzzer with stdin closed and the
-close_fd_mask=2 flag, and stderr will not be muted.

Example fuzzer:

extern "C" int LLVMFuzzerTestOneInput(const uint8_t* Data, size_t Size) {
  fprintf(stderr, "STDERR\n");
  fprintf(stdout, "STDOUT\n");
  return 0;
}

Invocation (muting both stderr and stdout):

./test -close_fd_mask=3 -runs=1 0<&-
INFO: Seed: 1155116940
INFO: Loaded 1 modules   (1 inline 8-bit counters): 1 [0x48b020, 0x48b021),
INFO: Loaded 1 PC tables (1 PCs): 1 [0x478dc8,0x478dd8),
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
STDERR
INFO: A corpus is not provided, starting from an empty corpus
STDERR
Done 2 runs in 0 second(s)

[phosek]

LGTM

compiler-rt/lib/fuzzer/FuzzerIO.cpp

 
 std::string DirPlusFile(const std::string &DirPath,
                         const std::string &FileName) {
   return DirPath + GetSeparator() + FileName;
 }
 
 void DupAndCloseStderr() {
   int OutputFd = DuplicateFile(2);
-  if (OutputFd > 0) {
+  if (OutputFd >= 0) {
     FILE *NewOutputFile = OpenFile(OutputFd, "w");
     if (NewOutputFile) {
       OutputFile = NewOutputFile;
       if (EF->__sanitizer_set_report_fd)
         EF->__sanitizer_set_report_fd(
             reinterpret_cast<void *>(GetHandleFromFd(OutputFd)));
       DiscardOutput(2);
     }