Fix Digest::SHA1#update with large input

[rhenium]

Digest::SHA1#update fails when a very large String is passed in a single call.

Passing 2**29 bytes (512 MB) or more at once does not update the message length counter correctly, which results in producing an incorrect output.

$ ruby -rdigest -e'd=Digest::SHA1.new; d<<"a"*(512*1024*1024); puts d.hexdigest'
40cd9c4b14e7b8d0940a3a92c8a7661fad85a821
$ ruby -rdigest -e'd=Digest::SHA1.new; 512.times{d<<"a"*(1024*1024)}; puts d.hexdigest'
0ea59bfe8787939816796610c73deb1c625e03ed
$ ruby -e'print "a"*(512*1024*1024)'|sha1sum
0ea59bfe8787939816796610c73deb1c625e03ed  -

Passing 2**32 bytes or more causes an infinite loop because the loop counter is too small.