crypto: fix bug in large buffer handling

crypto: fix bug in large buffer handling

Problem reported by Serge Belyshev for glibc in
<http://sourceware.org/bugzilla/show_bug.cgi?id=14090> and for gnulib in
<http://lists.gnu.org/archive/html/bug-gnulib/2012-05/msg00226.html>.
* lib/md4.c (md4_process_block):
* lib/md5.c (md5_process_block):
* lib/sha1.c (sha1_process_block):
* lib/sha256.c (sha256_process_block):
Don't assume the buffer length is less than 2**32.