ui-log: Prevent crash when given empty range search

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-log.c b/ui-log.c
index bc0c02c..b9771fa 100644
--- a/ui-log.c
+++ b/ui-log.c
@@ -162,7 +162,7 @@
 
 	argv[1] = disambiguate_ref(tip);
 
-	if (grep && pattern) {
+	if (grep && pattern && *pattern) {
 		if (!strcmp(grep, "grep") || !strcmp(grep, "author") ||
 		    !strcmp(grep, "committer"))
 			argv[argc++] = fmt("--%s=%s", grep, pattern);