[StaticAnalyzer] Improve+Refactor MallocOverflowSecurityChecker

[Aditya K]

Hi Anna,
Currently MallocOverflowSecurityChecker does not catch cases like: malloc(n * 0 * sizeof(int));
This patch reject such cases. Please give feedback/suggestions for improvement.

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20150504/128860.html

I have another patch which builds on the top of this one.
I'm planning to put that up for review once this gets in.



Thanks,
-Aditya