[analyzer; new edges] Don't crash if the top-level entry edge is missing.

We previously asserted that there was a top-level function entry edge, but
if the function decl's location is invalid (or within a macro) this edge
might not exist. Change the assertion to an actual check, and don't drop
the first path piece if it doesn't match.

<rdar://problem/14070304>

llvm-svn: 183358