signed / unsigned warnings in script.h and serialize.h #1541

This is the output, when compiling in Qt creator (Bitcoin-Qt) Warning:comparison between signed and unsigned integer expressions [-Wsign-compare]

serialize.h: https://github.com/bitcoin/bitcoin/blob/master/src/serialize.h#L812 https://github.com/bitcoin/bitcoin/blob/master/src/serialize.h#L824 https://github.com/bitcoin/bitcoin/blob/master/src/serialize.h#L837

script.h: https://github.com/bitcoin/bitcoin/blob/master/src/script.h#L455

If a dev can give a hint on how to fix the warnings, I'm willing to create a patch.

There are more warnings in master than just that, but you can see the actual warnings at http://jenkins.bluematt.me/job/Bitcoin/378/console

That are the only warnings left when compiling my local build, so these are the ones I could report :). Shouldn't be too hard to fix, but iterators and that stuff is not mine.