fix lua SPDEBUG NULL dereference on gbl_break_spname

gbl_break_spname can be NULL when debug_sp() sets it from an
uninitialized local. Add NULL guards before strcasecmp/strncasecmp
in db_debug() and InstructionCountHook() to prevent crash when
SET SPDEBUG ON is used without a fully established debug session.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mohitkhullar

lua/sp.c

@@ -1578,7 +1578,7 @@ static int db_debug(lua_State *lua)
             sprintf(sp_info, "%s:%s", sp->spname, sp->spversion.version_str);
         }
         len = strlen(sp_info);
-        if (strncasecmp(sp_info, gbl_break_spname, len) == 0) {
+        if (gbl_break_spname && strncasecmp(sp_info, gbl_break_spname, len) == 0) {
             gbl_break_lua = pthread_self();
         }
     }
@@ -2080,7 +2080,7 @@ static void InstructionCountHook(lua_State *lua, lua_Debug *debug)
                 sprintf(sp_info, "%s:%s:%d", sp->spname,
                         sp->spversion.version_str, debug->currentline);
             }
-            if (strcasecmp(sp_info, gbl_break_spname) == 0) {
+            if (gbl_break_spname && strcasecmp(sp_info, gbl_break_spname) == 0) {
                 gbl_break_lua = pthread_self();
             }
         }