[epiphany] Fix webextension critical message

[Michael Catanzaro <mcatanzaro src gnome org>]

commit a0897660671e681fa6c24725f2307b137cf1501e
Author: Jan-Michael Brummer <jan brummer tabos org>
Date:   Tue Feb 2 17:52:53 2021 +0100

    Fix webextension critical message
    
    Fixes ** (epiphany:25129): CRITICAL **: 17:34:25.183: WebKitUserScript* 
webkit_user_script_new_for_world(const gchar*, WebKitUserContentInjectedFrames, 
WebKitUserScriptInjectionTime, const char*, const gchar* const*, const gchar* const*): assertion 'source' 
failed

 src/webextension/ephy-web-extension-manager.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/src/webextension/ephy-web-extension-manager.c b/src/webextension/ephy-web-extension-manager.c
index 009168dee..bfed03f85 100644
--- a/src/webextension/ephy-web-extension-manager.c
+++ b/src/webextension/ephy-web-extension-manager.c
@@ -875,10 +875,13 @@ run_background_script (EphyWebExtensionManager *self,
 
     base_uri = g_strdup_printf ("webextension://%s/", ephy_web_extension_get_guid (web_extension));
     for (unsigned int i = 0; i < scripts->len; i++) {
-      char *script_file = g_ptr_array_index (scripts, i);
+      const char *script_file = g_ptr_array_index (scripts, i);
       g_autofree char *data = NULL;
       WebKitUserScript *user_script;
 
+      if (!script_file)
+        continue;
+
       data = ephy_web_extension_get_resource_as_string (web_extension, script_file);
       user_script = webkit_user_script_new_for_world (data,
                                                       WEBKIT_USER_CONTENT_INJECT_TOP_FRAME,