Avoid GC run between mysql_stmt_execute and mysql_stmt_store_result

Thanks to @kamipo for diagnosing the problem and drafting the first PR.

This fixes a regression caused by brianmario#912 due to calling `rb_funcall`
between `mysql_stmt_execute` and `mysql_stmt_store_result`, it will
cause `mysql_stmt_close` to be called in wrong order.

In further testing, `rb_hash_aref` can also call `rb_funcall` if the
query_options hash has a `default_proc` set, so adding a stronger
comment to remind future maintainers.

Fixes brianmario#956.

Author: sodabrew

ext/mysql2/statement.c

@@ -403,6 +403,22 @@ static VALUE rb_mysql_stmt_execute(int argc, VALUE *argv, VALUE self) {
     }
   }
 
+  // Duplicate the options hash, merge! extra opts, put the copy into the Result object
+  current = rb_hash_dup(rb_iv_get(stmt_wrapper->client, "@query_options"));
+  (void)RB_GC_GUARD(current);
+  Check_Type(current, T_HASH);
+
+  // Merge in hash opts/keyword arguments
+  if (!NIL_P(opts)) {
+    rb_funcall(current, intern_merge_bang, 1, opts);
+  }
+
+  is_streaming = (Qtrue == rb_hash_aref(current, sym_stream));
+
+  // From stmt_execute to mysql_stmt_result_metadata to stmt_store_result,
+  // no Ruby API calls are allowed. The GC may collect the Statement object,
+  // and not even RB_GC_GUARD seems to be able to protect it here.
+
   if ((VALUE)rb_thread_call_without_gvl(nogvl_stmt_execute, stmt, RUBY_UBF_IO, 0) == Qfalse) {
     FREE_BINDS;
     rb_raise_mysql2_stmt_error(stmt_wrapper);
@@ -421,17 +437,6 @@ static VALUE rb_mysql_stmt_execute(int argc, VALUE *argv, VALUE self) {
     return Qnil;
   }
 
-  // Duplicate the options hash, merge! extra opts, put the copy into the Result object
-  current = rb_hash_dup(rb_iv_get(stmt_wrapper->client, "@query_options"));
-  (void)RB_GC_GUARD(current);
-  Check_Type(current, T_HASH);
-
-  // Merge in hash opts/keyword arguments
-  if (!NIL_P(opts)) {
-    rb_funcall(current, intern_merge_bang, 1, opts);
-  }
-
-  is_streaming = (Qtrue == rb_hash_aref(current, sym_stream));
   if (!is_streaming) {
     // recieve the whole result set from the server
     if (mysql_stmt_store_result(stmt)) {