vec(): defer lvalue out-of-range croaking

RT #131083

Recent commits v5.25.10-81-gd69c430 and v5.25.10-82-g67dd6f3 added
out-of-range/overflow checks for the offset arg of vec().  However in
lvalue context, these croaks now happen before the SVt_PVLV was created,
rather than when its set magic was called. This means that something like

    sub f { $x = $_[0] }
    f(vec($s, -1, 8))

now croaks even though the out-of-range value never ended up getting used
in lvalue context.

This commit fixes things by, in pp_vec(), rather than croaking, just set
flag bits in LvFLAGS() to indicate that the offset is -Ve / out-of-range.

Then in Perl_magic_getvec(), return 0 if these flags are set, and in
Perl_magic_setvec() croak with a suitable error.

1 files changed, 2 insertions, 1 deletions

diff --git a/sv.h b/sv.h
index 82130b7fc0..5e9c5b6881 100644
--- a/sv.h
+++ b/sv.h
@@ -541,7 +541,8 @@ struct xpvlv {
     SV*		xlv_targ;
     char	xlv_type;	/* k=keys .=pos x=substr v=vec /=join/re
 				 * y=alem/helem/iter t=tie T=tied HE */
-    char	xlv_flags;	/* 1 = negative offset  2 = negative len */
+    char	xlv_flags;	/* 1 = negative offset  2 = negative len
+                                   4 = out of range (vec) */
 };
 
 #define xlv_targoff xlv_targoff_u.xlvu_targoff