embed.fnc/group_end: correct the patend pointer decorator

This was incorrectly asserting the supplied string had at least one
character, which could produce an assertion instead of a useful error
message for the user of pack().

Related to #23980

Author: tonycoz

embed.fnc

@@ -5306,7 +5306,7 @@ ST	|char	|first_symbol	|NN const char *pat			\
 RS	|const char *|get_num	|NN const char *patptr			\
 				|NN SSize_t *lenptr
 S	|const char *|group_end |SPTR const char *patptr		\
-				|EPTR const char *patend		\
+				|EPTRQ const char *patend		\
 				|char ender
 RS	|SV *	|is_an_int	|NN const char *s			\
 				|STRLEN l

proto.h

@@ -6,7 +6,7 @@ BEGIN {
     set_up_inc(qw '../lib ../cpan/Math-BigInt/lib');
 }
 
-plan tests => 14724;
+plan tests => 14726;
 
 use strict;
 use warnings qw(FATAL all);
@@ -2066,3 +2066,10 @@ SKIP:
 	like("$@", qr{Mismatched brackets in template},
 			"should match brackets correctly even without recursion");
 }
+
+{ # see discussion in #23980
+    ok !eval { my $x = pack "C["; 1 },
+      "pack throws with no closing ]";
+    like $@, qr/No group ending character '\]' found in template/,
+      "check the message";
+}