string: Fix unnecessary masking of memchr.

If input length parameter is decreased to zero, masking was
unnecessarily processed.
However it was harmless, since the mask was correctly set to zero.
Masking is required only in case of overread.

Changing the flag from HI to HS means, skip if zero flag is set.
HI = Unsigned higher: (C==1) && (Z==0)
HS = Unsigned higher or same: C==1