[modules] Fix findDirectiveAtLoc to not call a member function on a null pointer.
[modules] Fix findDirectiveAtLoc to not call a member function on a null pointer.

This is exercised by existing tests, and fixes a failure with -fsanitize=null.
No observable change otherwise; the code happened to do the right thing in
practice under recent versions of Clang and GCC because
MacroDirective::getDefinition happens to check whether this == null.


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@240691 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang