Fix trivial coverity warning
Merge pull request #922 from noakesmichael/cov-0

Fix trivial coverity warning

I recently touched bb.cpp and this caused Coverity to repeat a complaint about dead-code.
It took me a couple of looks to see the issue.  This is a loop over a std::vector<Expr*> and
the body tests whether the element is a NULL pointer or not; in practice this won't happen
for the vector in question but it must have seemed like a nice check when it was added.

However the macro for_vector() terminates early if it is finds a slot with NULL and so the
check is indeed dead-code.

I am not wholly convinced about this definition of for_vector but I think there may
be code that relies on this behavior.

Compiled: darwin/clang
Test: None; The change is trivial and the code is used to print the contents of BBs
Review: None