SvGROW: no overallocation

e.g. adding ".pm" to "warnings": 8 + 3 = 11. needing a len=12.
SvLEN is already 12, but sv_grow realloced to 8+3+3=14+1=15->16.

various people decided over time in the front and in the back to add just
another one for safety.  sv_grow adds the one for \0 already, so there's no
need in the front.  only the SvGROW macro logic, which checks against SvLEN.
But even this had it off-by-one.

memory numbers (DEBUGGING):
miniperl -e0            262890 => 262833 byte
miniperl -Ilib -E0      425636 => 423040 byte
perl Config ...         797578 => 796932 byte

$ PERL_HASH_SEED=0 valgrind --tool=massif --massif-out-file=massif.perl.p0
./perl -Ilib -MConfig -Mwarnings -Mstrict -V:ccflags ; grep -h mem_heap_B=
massif.perl.p0 |cut -c12-|sort -rn|head -n1

With #141 smallstrings the first goes down to 262689 bytes.

sv.c

@@ -5473,11 +5473,12 @@ Perl_sv_catpvn_flags(pTHX_ SV *const dsv, const char *sstr, const STRLEN slen, c
 	 sv_utf8_upgrade_flags_grow(dsv, 0, slen + 1);
 	 dlen = SvCUR(dsv);
       }
-      else SvGROW(dsv, dlen + slen + 3);
+      else
+          SvGROW(dsv, dlen + slen);
       if (sstr == dstr)
 	sstr = SvPVX_const(dsv);
       Move(sstr, SvPVX(dsv) + dlen, slen, char);
-      SvCUR_set(dsv, SvCUR(dsv) + slen);
+      SvCUR_set(dsv, dlen + slen);
     }
     else {
 	/* We inline bytes_to_utf8, to avoid an extra malloc. */
@@ -5489,7 +5490,7 @@ Perl_sv_catpvn_flags(pTHX_ SV *const dsv, const char *sstr, const STRLEN slen, c
 	   bytes *and* utf8, which would indicate a bug elsewhere. */
 	assert(sstr != dstr);
 
-	SvGROW(dsv, dlen + slen * 2 + 3);
+	SvGROW(dsv, dlen + slen * 2);
 	d = (U8 *)SvPVX(dsv) + dlen;
 
 	while (sstr < send) {

sv.h

@@ -2262,14 +2262,14 @@ See also C<L</PL_sv_yes>> and C<L</PL_sv_no>>.
 
 #ifdef PERL_ANY_COW
 # define SvGROW(sv,len) \
-        (SvIsCOW(sv) || SvLEN(sv) < ((len)+2) ? sv_grow(sv,(len)+2) : SvPVX(sv))
+    (SvIsCOW(sv) || SvLEN(sv) <= (len) ? sv_grow(sv,(len)) : SvPVX(sv))
 # define SvGROW_mutable(sv,len) \
-    (SvLEN(sv) < ((len)+2) ? sv_grow(sv,(len)+2) : SvPVX_mutable(sv))
+    (SvLEN(sv) <= (len) ? sv_grow(sv,(len)) : SvPVX_mutable(sv))
 #else
 # define SvGROW(sv,len) \
-    (SvLEN(sv) < ((len)+1) ? sv_grow(sv,(len)+1) : SvPVX(sv))
+    (SvLEN(sv) <= (len) ? sv_grow(sv,(len)) : SvPVX(sv))
 # define SvGROW_mutable(sv,len) \
-    (SvLEN(sv) < ((len)+1) ? sv_grow(sv,(len)+1) : SvPVX_mutable(sv))
+    (SvLEN(sv) <= (len) ? sv_grow(sv,(len)) : SvPVX_mutable(sv))
 #endif
 #define Sv_Grow sv_grow