Take advantage of clang-3.9 new nonnull attribute

[thk123]

Builds had started failing on Travis due to an upgrade to clang-3.9:

https://travis-ci.org/diffblue/cbmc/jobs/274155578#L799

Spotted by @smowton

[reuk]

LGTM but IMO the .travis.yml should also be updated to use clang-3.9 explicitly

[kroening]

Already done by #1364.

[smowton]

@kroening compare #1369, which ignores the warning as not applicable to a library solely for CBMC's internal consumption.

It's a shame to remove if(s1!=0 && s1==s2) just because GCC/Clang would optimise that check away. Clearly CBMC would never do so, since null accesses and other undefined behaviour are always interesting to its bug-checking mission.

[smowton]

This version on the other hand is wrong I think, since it will silently accept strcmp(nullptr, nullptr), which should yield a fault report from CBMC.

[thk123]

As pointed out, this is addressed (better) in other PRs.

[kroening]

@smowton The idea is the following: the optimisation can be put back in once we add a precondition that says that the parameters must point to zero-terminated strings.