Bug 700583 - Null-check our way out of crashes on both mobile and desktop in pr_FindSymbolInLib caused by plugin code where the plugin fails to load correctly, r=josh

dom/plugins/base/nsPluginsDirUnix.cpp

@@ -307,6 +307,10 @@ nsresult nsPluginFile::LoadPlugin(PRLibrary **outLibrary)
     printf("LoadPlugin() %s returned %lx\n", 
            libSpec.value.pathname, (unsigned long)pLibrary);
 #endif
+
+    if (!pLibrary) {
+        return NS_ERROR_FAILURE;
+    }
     
     return NS_OK;
 }