Fix segfault when accessing fields/field_types on freed result

Add resultFreed check to rb_mysql_result_fetch_fields and
rb_mysql_result_fetch_field_types before accessing wrapper->result.

The segfault occurs when:
1. A query returns 0 rows
2. Internal row caching iterates (0 iterations), never populating wrapper->fields
3. Result is freed after iteration completes
4. .fields is called, wrapper->fields is Qnil
5. mysql_num_fields(wrapper->result) accesses freed memory -> SEGFAULT

This check already exists in other functions (e.g., rb_mysql_result_each_)
but was missing in the fields accessor functions.

Based on fix for #1426

Author: Jonathan Powell

ext/mysql2/result.c

@@ -939,6 +939,9 @@ static VALUE rb_mysql_result_fetch_fields(VALUE self) {
   }
 
   if (wrapper->fields == Qnil) {
+    if (wrapper->resultFreed) {
+      rb_raise(cMysql2Error, "Result set has already been freed");
+    }
     wrapper->numberOfFields = mysql_num_fields(wrapper->result);
     wrapper->fields = rb_ary_new2(wrapper->numberOfFields);
   }
@@ -958,6 +961,9 @@ static VALUE rb_mysql_result_fetch_field_types(VALUE self) {
   GET_RESULT(self);
 
   if (wrapper->fieldTypes == Qnil) {
+    if (wrapper->resultFreed) {
+      rb_raise(cMysql2Error, "Result set has already been freed");
+    }
     wrapper->numberOfFields = mysql_num_fields(wrapper->result);
     wrapper->fieldTypes = rb_ary_new2(wrapper->numberOfFields);
   }