git_mkstemps: correctly test return value of open()

open() returns -1 on failure, and indeed 0 is a possible success value
if the user closed stdin in our process.  Fix the test.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -322,7 +322,7 @@ int git_mkstemps_mode(char *pattern, int suffix_len, int mode)
 		template[5] = letters[v % num_letters]; v /= num_letters;
 
 		fd = open(pattern, O_CREAT | O_EXCL | O_RDWR, mode);
-		if (fd > 0)
+		if (fd >= 0)
 			return fd;
 		/*
 		 * Fatal error (EPERM, ENOSPC etc).