utils: avoid segfault when calling osmo_strlcpy(src=NULL)

Change-Id: Ieba7ba262ace2e370a4b9a550b3131fb13f07413
diff --git a/src/utils.c b/src/utils.c
index ef8e916..f5894d8 100644
--- a/src/utils.c
+++ b/src/utils.c
@@ -372,7 +372,7 @@
  */
 size_t osmo_strlcpy(char *dst, const char *src, size_t siz)
 {
-	size_t ret = strlen(src);
+	size_t ret = src ? strlen(src) : 0;
 
 	if (siz) {
 		size_t len = (ret >= siz) ? siz - 1 : ret;