MXS-2095: Add runtime detection of unknown SQL

If the query statement is wrongly treated as a table creation statement it
could cause a crash. To handle this, unknown SQL is now reported and the
processing is stopped early. This does not solve the root cause of the
problem but makes it possible to detect it in the future.

server/modules/routing/avrorouter/avro_schema.c

@@ -712,6 +712,12 @@ TABLE_CREATE* table_create_alloc(const char* ident, const char* sql, int len)
     const char* statement_sql = get_table_definition(sql, len, &stmt_len);
     ss_dassert(statement_sql);
 
+    if (!statement_sql)
+    {
+        MXS_ERROR("Cannot process SQL: %.*s", len, sql);
+        return NULL;
+    }
+
     char* tbl_start = strchr(ident, '.');
     ss_dassert(tbl_start);
     *tbl_start++ = '\0';