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/evalfunc.c b/src/evalfunc.c
index 24c3194..7325f5d 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -13090,10 +13090,13 @@
 		save_starting = -1;
 	    }
 	}
+	else if (STRCMP(name, (char_u *)"nfa_fail") == 0)
+	    nfa_fail_for_testing = val;
 	else if (STRCMP(name, (char_u *)"ALL") == 0)
 	{
 	    disable_char_avail_for_testing = FALSE;
 	    disable_redraw_for_testing = FALSE;
+	    nfa_fail_for_testing = FALSE;
 	    if (save_starting >= 0)
 	    {
 		starting = save_starting;