[analyzer] Fix crash in MallocChecker.

Fix a crash in the MallocChecker when the extent size for the argument
to new[] is not known.

A patch by Abramo Bagnara and Dániel Krupp!

https://reviews.llvm.org/D27849

Differential Revision: https://reviews.llvm.org/D27849

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289970 91177308-0d34-0410-b5e6-96231b3b80d8