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.

src/vim.h

@@ -1013,6 +1013,7 @@ extern int (*dyn_libintl_putenv)(const char *envstring);
 /* values for reg_do_extmatch */
 # define REX_SET	1	/* to allow \z\(...\), */
 # define REX_USE	2	/* to allow \z\1 et al. */
+# define REX_ALL	(REX_SET | REX_USE)
 #endif
 
 /* Return values for fullpathcmp() */