MDEV-22130 SHOW WARNINGS will SIGSEGV 10.5 optimized build after setting...

MDEV-22130 SHOW WARNINGS will SIGSEGV 10.5 optimized build after setting CHARACTER_SET_RESULTS to NULL and running any invalid SQL | Binary_string::copy_printable_hhhh

The old code did not take into account that
thd->variables.character_set_results can be NULL.

mysql-test/main/ctype_errors.result

@@ -49,3 +49,14 @@ ERROR 42000: \0423 \0432\0430\0441 \043E\0448\0438\0431\043A\0430 \0432 \0437\04
 disconnect con1;
 connection default;
 End of 5.5 tests
+#
+# MDEV-22130 SHOW WARNINGS will SIGSEGV 10.5 optimized build after setting CHARACTER_SET_RESULTS to NULL and running any invalid SQL | Binary_string::copy_printable_hhhh
+#
+SET NAMES latin1;
+SET @@CHARACTER_SET_RESULTS=NULL;
+a;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'a' at line 1
+SHOW WARNINGS;
+Level	Code	Message
+Error	1064	You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'a' at line 1
+SET NAMES latin1;

mysql-test/main/ctype_errors.test

@@ -58,3 +58,14 @@ disconnect con1;
 connection default;
 
 --echo End of 5.5 tests
+
+--echo #
+--echo # MDEV-22130 SHOW WARNINGS will SIGSEGV 10.5 optimized build after setting CHARACTER_SET_RESULTS to NULL and running any invalid SQL | Binary_string::copy_printable_hhhh
+--echo #
+
+SET NAMES latin1;
+SET @@CHARACTER_SET_RESULTS=NULL;
+--error ER_PARSE_ERROR
+a;
+SHOW WARNINGS;
+SET NAMES latin1;

sql/protocol.cc

@@ -1194,7 +1194,7 @@ bool Protocol::store_warning(const char *from, size_t length)
 {
   BinaryStringBuffer<MYSQL_ERRMSG_SIZE> tmp;
   CHARSET_INFO *cs= thd->variables.character_set_results;
-  if (cs == &my_charset_bin)
+  if (!cs || cs == &my_charset_bin)
     cs= system_charset_info;
   if (tmp.copy_printable_hhhh(cs, system_charset_info, from, length))
     return net_store_data((const uchar*)"", 0);