SvPV_shrink_to_cur: don't be unrealistic, do 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.

The macro has also been modified such that:
 * No reallocation will be attempted if the saving is unrealistically
   small, or otherwise no saving is likely to be achievable.
 * The intended saving is compared with SvLEN(sv), which enables checks
   at call sites to be simplified.

Author: richardleach

sv.h

@@ -1590,9 +1590,19 @@ 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: Ensure the buffer is big enough to be COWed in the future, so
+          + 1 for the trailing null byte + 1 for the COW count.
+ * Don't try to shrink if the saving is too small to be realistic (given
+ * the realities of allocators) or noticeable.
+ * 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 ((SvLEN(sv) > _lEnGtH + PTRSIZE) &&  \
+                       ( _lEnGtH >= PERL_STRLEN_NEW_MIN) ) \
+                       SvPV_renew(sv, _lEnGtH);            \
                  } STMT_END
 
 /*