Stop minlen regexp optimisation from rejecting long strings

This fixes #112790 and part of #116907.

The length of the string is cast to I32, so it wraps and end up less
than the minimum length.

For now, simply skip this optimisation if minlen itself wraps and
becomes negative.

MANIFEST

@@ -4939,6 +4939,7 @@ t/base/while.t			See if while work
 t/benchmark/rt26188-speed-up-keys-on-empty-hash.t	Benchmark if keys on empty hashes is fast enough
 t/bigmem/pos.t			Check that pos() handles large offsets
 t/bigmem/read.t			Check read() handles large offsets
+t/bigmem/regexp.t		Test regular expressions with large strings
 t/bigmem/vec.t			Check vec() handles large offsets
 t/cmd/elsif.t			See if else-if works
 t/cmd/for.t			See if for loops work

pp_hot.c

@@ -1383,7 +1383,7 @@ PP(pp_match)
 	rx = PM_GETRE(pm);
     }
 
-    if (RX_MINLEN(rx) > (I32)len) {
+    if (RX_MINLEN(rx) >= 0 && (STRLEN)RX_MINLEN(rx) > len) {
         DEBUG_r(PerlIO_printf(Perl_debug_log, "String shorter than min possible regex match\n"));
 	goto nope;
     }

t/bigmem/regexp.t

@@ -0,0 +1,22 @@
+#!perl
+BEGIN {
+    chdir 't';
+    unshift @INC, "../lib";
+    require './test.pl';
+}
+
+use Config qw(%Config);
+
+$ENV{PERL_TEST_MEMORY} >= 2
+    or skip_all("Need ~2Gb for this test");
+$Config{ptrsize} >= 8
+    or skip_all("Need 64-bit pointers for this test");
+
+plan(2);
+
+# [perl #116907]
+# ${\2} to defeat constant folding, which in this case actually slows
+# things down
+my $x=" "x(${\2}**31);
+ok $x =~ /./, 'match against long string succeeded';
+is "$-[0]-$+[0]", '0-1', '@-/@+ after match against long string';