Check for ao2_alloc failure in __ast_channel_internal_alloc.

Fix a crash that could occur in __ast_channel_internal_alloc if
ao2_alloc fails.

ASTERISK-24991 #close

Change-Id: I4ca89189eb22f907408cb87d0a1645cfe1314a90

main/channel_internal_api.c

@@ -1449,6 +1449,10 @@ struct ast_channel *__ast_channel_internal_alloc(void (*destructor)(void *obj),
 	tmp = ao2_alloc(sizeof(*tmp), destructor);
 #endif
 
+	if (!tmp) {
+		return NULL;
+	}
+
 	if ((ast_string_field_init(tmp, 128))) {
 		return ast_channel_unref(tmp);
 	}