Fix invalid read in find_bytes().

Validate arguments. Move the conditional to the start of the loop
to avoid an invalid read if len < slen.

Closes issue #21.
Disable tests which are hanging with valgrind.

find_bytes() rejecting invalid arguments hangs some valgrind tests.
They appear to be segfaulting inside std:vector faster than valgrind
can process the signals.

This should be re-enabled when the problem is resolved.
Merge pull request #131 from rillian/issue-39

Fix find_bytes and disable hanging unit tests.