[COVERITY] Fix (error case) file handle leak in util/subst program

Need to close old_f before returning since it had been successfully opened
before.

Coverity ID: 19: Resource Leak

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
diff --git a/util/subst.c b/util/subst.c
index 445d318..fadf328 100644
--- a/util/subst.c
+++ b/util/subst.c
@@ -279,8 +279,10 @@
 	if (!old_f)
 		return 0;
 	new_f = fopen(newfn, "r");
-	if (!new_f)
+	if (!new_f) {
+		fclose(old_f);
 		return 0;
+	}
 
 	while (1) {
 		oldcp = fgets(oldbuf, sizeof(oldbuf), old_f);