[analyzer] Don't assert when combining using .* on a temporary.

[analyzer] Don't assert when combining using .* on a temporary.

While we don't model pointer-to-member operators yet (neither .* nor ->*),
CallAndMessageChecker still checks to make sure the 'this' object is not
null or undefined first. However, it also expects that the object should
always have a valid MemRegion, something that's generally important elsewhere
in the analyzer as well. Ensure this is true ahead of time, just like we do
for member access.

PR19531

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207561 91177308-0d34-0410-b5e6-96231b3b80d8