config-parser: add missing return for an error case

Fixes
config-parser.c: In function 'handle_key':
config-parser.c:81: error: control reaches end of non-void function

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

shared/config-parser.c

@@ -78,6 +78,8 @@ handle_key(const struct config_key *key, const char *value)
 		assert(0);
 		break;
 	}
+
+	return -1;
 }
 
 int