btrfs-progs: image: fix bogus check after cpu on-line detection

Comparing unsigned type for <= 0 does not make much sense, we should
really check the signed value returned by sysconf.

Resolves-coverity-id: 1324536
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs-image.c

@@ -2787,9 +2787,11 @@ int main(int argc, char *argv[])
 
 	if (compress_level > 0 || create == 0) {
 		if (num_threads == 0) {
-			num_threads = sysconf(_SC_NPROCESSORS_ONLN);
-			if (num_threads <= 0)
-				num_threads = 1;
+			long tmp = sysconf(_SC_NPROCESSORS_ONLN);
+
+			if (tmp <= 0)
+				tmp = 1;
+			num_threads = tmp;
 		}
 	} else {
 		num_threads = 0;