str: preallocate "-" string
str: preallocate "-" string

Since this is a relatively common string we can preallocate it and return it
instead of going to the heap.

One of the consumers of libjeffpc saw almost 1.5% reduction in struct str
allocations during startup:

cache                        buf     buf     buf     buf  memory     alloc alloc
name                        size  in use  in ptc   total  in use   succeed  fail
------------------------- ------ ------- ------- ------- ------- --------- -----
str-cache (before)            24    5347       -    5577    132K    652339     0
str-cache (after)             24    5347       -    5577    132K    642691     0

Note: This consumer is a heavy user of strings.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc