PATCH: [perl #126404] Assertion fail in (?[...])

This was due to the '()' in the test case being empty, and not reducing
to an operand.  The code didn't expect it, but was guarded by an
assertion.  Now it generates a proper error.

regcomp.c

@@ -13807,8 +13807,12 @@ S_handle_regex_sets(pTHX_ RExC_state_t *pRExC_state, SV** return_invlist,
                 /* Having gotten rid of the fence, we pop the operand at the
                  * stack top and process it as a newly encountered operand */
                 current = av_pop(stack);
-                assert(IS_OPERAND(current));
-                goto handle_operand;
+                if (IS_OPERAND(current)) {
+                    goto handle_operand;
+                }
+
+                RExC_parse++;
+                goto bad_syntax;
 
             case '&':
             case '|':

t/re/reg_mesg.t

@@ -260,6 +260,7 @@ my @death =
  '/((?# This is a comment in the middle of a token)*FAIL)/' => 'In \'(*VERB...)\', the \'(\' and \'*\' must be adjacent {#} m/((?# This is a comment in the middle of a token)*{#}FAIL)/',
  '/(?[\ &!])/' => 'Incomplete expression within \'(?[ ])\' {#} m/(?[\ &!{#}])/',    # [perl #126180]
  '/(?[()-!])/' => 'Incomplete expression within \'(?[ ])\' {#} m/(?[()-!{#}])/',    # [perl #126204]
+ '/(?[!()])/' => 'Incomplete expression within \'(?[ ])\' {#} m/(?[!(){#}])/',      # [perl #126404]
 );
 
 # These are messages that are warnings when not strict; death under 'use re