posix: Fix StdioFileHandle() for GCC

With GCC 6.3:

util/file/file_io_posix.cc: In function ‘crashpad::FileHandle crashpad::StdioFileHandle(crashpad::StdioStream)’:
util/file/file_io_posix.cc:193:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

Bug: crashpad:30
Change-Id: I03111b672ab7f796103ef61ea3d126fc25571390
Reviewed-on: https://chromium-review.googlesource.com/456820
Reviewed-by: Robert Sesek <rsesek@chromium.org>

util/file/file_io_posix.cc

@@ -190,6 +190,9 @@ FileHandle StdioFileHandle(StdioStream stdio_stream) {
     case StdioStream::kStandardError:
       return STDERR_FILENO;
   }
+
+  NOTREACHED();
+  return kInvalidFileHandle;
 }
 
 }  // namespace crashpad