Bug 1909771 - Avoid UndefinedBehavior mailnews/compose/src/nsMsgCompose.cpp. r=babolivier

Differential Revision: https://phabricator.services.mozilla.com/D217997

--HG--
extra : amend_source : f03fe441e91ab54fa13aa0d05090da61cbfc6bbf

mailnews/compose/src/nsMsgCompose.cpp

@@ -1922,7 +1922,12 @@ nsresult nsMsgCompose::CreateMessage(const nsACString& originalMsgURI,
       }
     }
     isFirstPass = false;
-    uri = nextUri + 1;
+    if (nextUri) {
+      // `nextUri` can be a null pointer if `strstr` did not find `://` in the
+      // URI earlier. Only increment it if that is not the case, to avoid
+      // undefined behaviors.
+      uri = nextUri + 1;
+    }
   } while (nextUri);
   PR_Free(uriList);
   return rv;