avformat/url: check return value of strchr

fix ticket: 8687
workflow should return if there have no value of strchr

Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavformat/url.c

@@ -90,6 +90,8 @@ static void trim_double_dot_url(char *buf, const char *rel, int size)
     if (p && (sep = strstr(p, "://"))) {
         sep += 3;
         root = strchr(sep, '/');
+        if (!root)
+            return;
     }
 
     /* set new current position if the root node is changed */
@@ -150,6 +152,7 @@ void ff_make_absolute_url(char *buf, int size, const char *base,
     }
     /* If rel actually is an absolute url, just copy it */
     if (!base || strstr(rel, "://") || rel[0] == '/') {
+        memset(buf, 0, size);
         trim_double_dot_url(buf, rel, size);
         return;
     }
@@ -177,6 +180,8 @@ void ff_make_absolute_url(char *buf, int size, const char *base,
         if (sep) {
             sep += 3;
             root = strchr(sep, '/');
+            if (!root)
+                return;
         }
     }