Use unsigned comparisons. Prior to this commit, SHA1_Update and

Use unsigned comparisons.  Prior to this commit, SHA1_Update and
RIPEMD160_Update were broken when all of the following conditions
applied:
(1) The platform is i386.
(2) The program calling *_Update is statically linked to libmd.
(3) The buffer provided to *_Update is aligned modulo 4 bytes.
(4) The buffer extends beyond 2GB.

Due to the design of this code, SHA1_Update and RIPEMD160_Update will
still be broken if conditions (1)-(3) apply AND the buffer extends
beyond 4GB (i.e., there is an integer overflow in computing "data + len").
Since this remaining bug simply replaces SIGSEGV with a bogus hash (and
non-broken programs should never provide such operands) I don't consider
it to be a serious problem.

MFC After:      1 week
PR:             kern/102795