sv_setsv_cow: apply the same tests we do for a normal COW copy

Previously if you had a successful match against an SV with a
SvLEN() large relative to the SvCUR() the regexp engine would
use sv_setsv_cow() to make a COW copy of the matched SV,
extending the life of the large allocation buffer.

A normal sv_setsv() normally didn't do such a COW copy, but the above
also marked the source SV as COW, so further copies of the SV
could even further extend the lifetime of the buffer, eg:

  while (<>) { # readline tends to make large SvLEN()
    /something/; # some sort of match
    push @save, $_; # with a successful match, the large $_ buffer
                    # survives until @save is released
  }

Fixes part of Perl#21877

sv.c

@@ -4912,6 +4912,12 @@ Perl_sv_setsv_cow(pTHX_ SV **pdsv, SV *ssv)
                     sv_dump(dsv);
     }
 #endif
+    if (!CHECK_COWBUF_THRESHOLD(cur, len)
+        || ! CHECK_COW_THRESHOLD(cur, len)) {
+        DEBUG_C(PerlIO_printf(Perl_debug_log,
+                              "Fast copy on write: Sizes not appropriate to COW\n"));
+        return FALSE;
+    }
     if (dsv) {
         if (SvTHINKFIRST(dsv))
             sv_force_normal_flags(dsv, SV_COW_DROP_PV);