patch 8.1.0098: segfault when pattern with \z() is very slow

Problem:    Segfault when pattern with \z() is very slow.
Solution:   Check for NULL regprog.  Add "nfa_fail" to test_override() to be
            able to test this.  Fix that 'searchhl' resets called_emsg.
diff --git a/src/globals.h b/src/globals.h
index db3a73f..8750859 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1634,6 +1634,7 @@
 /* flags set by test_override() */
 EXTERN int  disable_char_avail_for_testing INIT(= 0);
 EXTERN int  disable_redraw_for_testing INIT(= 0);
+EXTERN int  nfa_fail_for_testing INIT(= 0);
 
 EXTERN int  in_free_unref_items INIT(= FALSE);
 #endif