Add a check to avoid segfault

Triggered by out of bound array access when retreiving the mysql encoding.

Author: intrip

ext/mysql2/mysql_enc_to_ruby.h

@@ -244,3 +244,4 @@ const char *mysql2_mysql_enc_to_rb[] = {
   "UTF-8"
 };
 
+#define CHARSETNR_SIZE (sizeof(mysql2_mysql_enc_to_rb)/sizeof(mysql2_mysql_enc_to_rb[0]))

ext/mysql2/result.c

@@ -158,7 +158,7 @@ static VALUE mysql2_set_field_string_encoding(VALUE val, MYSQL_FIELD field, rb_e
     const char *enc_name;
     int enc_index;
 
-    enc_name = mysql2_mysql_enc_to_rb[field.charsetnr-1];
+    enc_name = (field.charsetnr-1 < CHARSETNR_SIZE) ? mysql2_mysql_enc_to_rb[field.charsetnr-1] : NULL;
     if (enc_name != NULL) {
       /* use the field encoding we were able to match */
       enc_index = rb_enc_find_index(enc_name);