sv_grow: performance improvement for short strings

Empty COW strings with CUR=0 ended up allocated as LEN=10.
Now they are rounded up to 4 or 8.

Timings:
+0   16.394324103 0.27%
+2   16.114379842 0.01%
+4   16.305622265 1.03%
+8   16.337438609 1.30%
+10  16.675009468 0.59%
with LD_LIBRARY_PATH=`pwd` perf stat -r2 ./perl t/harness t/op/*.t

+2 was consistently the best number, and +10 the worst.