http: fix nul deref on memcap reached

HttpRangeOpenFileAux may return NULL in different cases, including
when memcap is reached.
But is only caller did not check it before calling HttpRangeAppendData
which would dereference the NULL value.

Ticket: 7029

src/app-layer-htp-range.c

@@ -351,8 +351,10 @@ static HttpRangeContainerBlock *HttpRangeOpenFile(HttpRangeContainerFile *c, uin
 {
     HttpRangeContainerBlock *r =
             HttpRangeOpenFileAux(c, start, end, total, sbcfg, name, name_len, flags);
-    if (HttpRangeAppendData(sbcfg, r, data, len) < 0) {
-        SCLogDebug("Failed to append data while opening");
+    if (r) {
+        if (HttpRangeAppendData(sbcfg, r, data, len) < 0) {
+            SCLogDebug("Failed to append data while opening");
+        }
     }
     return r;
 }