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.

Additionally, eagerly populate fields when a result is created (for
non-streaming results) to prevent the issue from occurring at all.
This ensures fields are fetched before any iteration can free the
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);
   }
@@ -1192,6 +1198,7 @@ VALUE rb_mysql_result_to_obj(VALUE client, VALUE encoding, VALUE options, MYSQL_
   wrapper->lastRowProcessed = 0;
   wrapper->resultFreed = 0;
   wrapper->result = r;
+  wrapper->numberOfFields = 0;
   wrapper->fields = Qnil;
   wrapper->fieldTypes = Qnil;
   wrapper->rows = Qnil;
@@ -1221,6 +1228,14 @@ VALUE rb_mysql_result_to_obj(VALUE client, VALUE encoding, VALUE options, MYSQL_
    * should be processed here. */
   wrapper->is_streaming = (rb_hash_aref(options, sym_stream) == Qtrue ? 1 : 0);
 
+  /* Eagerly populate fields for non-streaming results to prevent
+   * segfault/error when accessing .fields on 0-row results that get freed
+   * after iteration completes but before .fields is accessed.
+   * See: https://github.com/brianmario/mysql2/issues/1426 */
+  if (r != NULL && !wrapper->is_streaming) {
+    rb_mysql_result_fetch_fields(obj);
+  }
+
   return obj;
 }