gsturi: Don't treat invalid format of uri as critical error

Normally uri is get from user input and invalid user input should not be treated as critical error. Moved gst_uri_is_valid outside of g_return_val_if_fail.

NULL uri is checked inside of gst_uri_is_valid and is correctly treated as critical error, removed unneeded checks of NULL uri outside of gst_uri_is_valid function.