Allow sdsrange to make a string with length 0.

[iveqy]

sdsrange(str, 0, -2) should result in an empty string.

For example

sds str = sdsnew("hello"); while (sdslen(str) > 0) str = sdsrange(str, 0, -2);

Would result in an inifinty loop before but will work as expected after this patch.