lib: avoid double free in loopdev.c

Found with scan-build.

Reviewed-by: Yuriy M. Kaminskiy <yumkam@gmail.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

lib/loopdev.c

@@ -460,6 +460,7 @@ static int loop_scandir(const char *dirname, int **ary, int hasprefix)
 			tmp = realloc(*ary, arylen * sizeof(int));
 			if (!tmp) {
 				free(*ary);
+				*ary = NULL;
 				closedir(dir);
 				return -1;
 			}