fix integer overflow in S_study_chunk().

It was calculating final_minlen + delta even when delta was already
SSize_t_MAX and final_minlen > 0.

This triggered it: /a(??{}){2}/.

Found by -fsanitize=undefined:

regcomp.c:5623:89: runtime error: signed integer overflow: 1 + 9223372036854775807 cannot be represented in type 'long'

1 files changed, 5 insertions, 2 deletions

diff --git a/regcomp.c b/regcomp.c
index c2521a97cc..4f5753a27b 100644
--- a/regcomp.c
+++ b/regcomp.c
@@ -5647,8 +5647,11 @@ PerlIO_printf(Perl_debug_log, "LHS=%"UVuf" RHS=%"UVuf"\n",
     {
         SSize_t final_minlen= min < stopmin ? min : stopmin;
 
-        if (!(RExC_seen & REG_UNBOUNDED_QUANTIFIER_SEEN) && (RExC_maxlen < final_minlen + delta)) {
-            RExC_maxlen = final_minlen + delta;
+        if (!(RExC_seen & REG_UNBOUNDED_QUANTIFIER_SEEN)) {
+            if (final_minlen > SSize_t_MAX - delta)
+                RExC_maxlen = SSize_t_MAX;
+            else if (RExC_maxlen < final_minlen + delta)
+                RExC_maxlen = final_minlen + delta;
         }
         return final_minlen;
     }