In Perl_sv_pos_u2b_flags and S_sv_pos_u2b_cached, return early for offset 0.

0 Unicode characters are always 0 octets long. Returning early ensures that
any offsets we calculate later will always be non-zero.
(cherry picked from commit 48f9cf718354a5326f9e9d40d02a063952160024,
as this is a prerequisite for 3e2d381)

diff --git a/sv.c b/sv.c
index c80db98..8a4e964 100644 (file)
--- a/sv.c
+++ b/sv.c
@@ -6157,6 +6157,9 @@ S_sv_pos_u2b_cached(pTHX_ SV *const sv, MAGIC **const mgp, const U8 *const start
 
     assert (uoffset >= uoffset0);
 
+    if (!uoffset)
+       return 0;
+
     if (!SvREADONLY(sv)
        && PL_utf8cache
        && (*mgp || (SvTYPE(sv) >= SVt_PVMG &&
@@ -6286,7 +6289,9 @@ Perl_sv_pos_u2b_flags(pTHX_ SV *const sv, STRLEN uoffset, STRLEN *const lenp,
        MAGIC *mg = NULL;
        boffset = sv_pos_u2b_cached(sv, &mg, start, send, uoffset, 0, 0);
 
-       if (lenp) {
+       if (lenp
+           && *lenp /* don't bother doing work for 0, as its bytes equivalent
+                       is 0, and *lenp is already set to that.  */) {
            /* Convert the relative offset to absolute.  */
            const STRLEN uoffset2 = uoffset + *lenp;
            const STRLEN boffset2