Perl_vnewSVpvf - use the pattern length to size the new PV buffer

This function currently creates a new SVt_PV with a minimum string
length buffer via:

    sv = newSV(1);

Simple measurements from compiling perl show that this is often sufficient.

When it isn't, the final buffer length is often close to strlen(pat).
Common cases that require a realloc() now will not with this commit. In
other cases, realloc()s will still be required, but the number will
likely be smaller by one.

Although min length & alloc behaviour can vary across platforms and
compilers, the following numbers are likely indicative:

|strlen(pat)|Inital SvLEN before|Initial SvLEN now|Final SvLEN|
|-----------|-------------------|-----------------|-----------|
|004|16|016|016|
|058|16|060|060|
|071|16|073|120|
|104|16|106|106|
|104|16|106|200|
|122|16|124|124|
|123|16|125|125|
|123|16|125|168|
|377|16|379|379|

Author: richardleach

sv.c

@@ -10009,9 +10009,11 @@ Perl_vnewSVpvf(pTHX_ const char *const pat, va_list *const args)
 
     PERL_ARGS_ASSERT_VNEWSVPVF;
 
-    sv = newSV(1);
+    STRLEN patlen = strlen(pat);
+
+    sv = newSV(patlen);
     SvPVCLEAR_FRESH(sv);
-    sv_vcatpvfn_flags(sv, pat, strlen(pat), args, NULL, 0, NULL, 0);
+    sv_vcatpvfn_flags(sv, pat, patlen, args, NULL, 0, NULL, 0);
     return sv;
 }