SvPV_shrink_to_cur: include space for COW

The `SvPV_shrink_to_cur` macro shrinks an allocation to `SvCUR(sv) + 1`,
which does not include an additional byte for Copy-On-Write (COW).

GH#22116 - a902d92 - short-circuited
constant folding on CONST OPs, as this should be unnecessary. However,
Dave Mitchell noticed that it had the inadvertent effect of disabling
COW on SVs holding UTF8 string literals (e.g. `"\x{100}abcd"`).

When a CONST OP is created, `Perl_ck_svconst` should mark its SV as
being COW-able. But SVs built via `S_scan_const`, when that has
called `SvPV_shrink_to_cur`, have resulting `SvLEN(sv)` values that
fail the `SvCANCOW(sv)` test. Previously, constant folding had the
effect of copying the literal into a buffer large enough for COW.

This commit modifies `SvPV_shrink_to_cur` to: allocate an additional
byte to allow for subsequent COWing directly.

Additionally, this macro will no longer try to shrink the buffer
below PERL_STRLEN_NEW_MIN, since that is likely to have no effect.

Author: richardleach

sv.h

@@ -1590,9 +1590,14 @@ L</C<SV_CHECK_THINKFIRST_COW_DROP>> before calling this.
 =cut
 */
 
-#define SvPV_shrink_to_cur(sv) STMT_START { \
-                   const STRLEN _lEnGtH = SvCUR(sv) + 1; \
-                   SvPV_renew(sv, _lEnGtH); \
+/* Notes: Make sure the buffer is big enough to be COWed in the future.
+ * Don't try to shrink smaller than PERL_STRLEN_NEW_MIN, as that is
+ * unlikely to result in a smaller allocation. */
+
+#define SvPV_shrink_to_cur(sv) STMT_START {              \
+                   const STRLEN _lEnGtH = SvCUR(sv) + 2; \
+                   if (PERL_STRLEN_NEW_MIN < _lEnGtH)    \
+                       SvPV_renew(sv, _lEnGtH);          \
                  } STMT_END
 
 /*