patch 8.2.3883: crash when switching to other regexp engine fails

Problem:    Crash when switching to other regexp engine fails.
Solution:   Check for regprog being NULL.

src/ex_cmds.c

@@ -4975,6 +4975,8 @@ ex_global(exarg_T *eap)
 	    // a match on this line?
 	    match = vim_regexec_multi(&regmatch, curwin, curbuf, lnum,
 						       (colnr_T)0, NULL, NULL);
+	    if (regmatch.regprog == NULL)
+		break;  // re-compiling regprog failed
 	    if ((type == 'g' && match) || (type == 'v' && !match))
 	    {
 		ml_setmarked(lnum);