Sema: Fix null dereference in overApproximateAvailabilityAtLocation().

The `TypeRefinementContext::findMostRefinedSubContext()` method can return
`nullptr` in some circumstances, which means that callers need to check the
result before dereferencing it in order to avoid a crash. I haven't introduced
a test case because I can only reproduce the circumstances under which
`findMostRefinedSubContext()` returns `nullptr` with a specific project that is
exercising a bug in Swift macros. The fix here is obvious enough hardening,
though.

Resolves rdar://125054962

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -1375,9 +1375,11 @@ TypeChecker::overApproximateAvailabilityAtLocation(SourceLoc loc,
     if (rootTRC) {
       TypeRefinementContext *TRC =
           rootTRC->findMostRefinedSubContext(loc, Context);
-      OverApproximateContext.constrainWith(TRC->getAvailabilityInfo());
-      if (MostRefined) {
-        *MostRefined = TRC;
+      if (TRC) {
+        OverApproximateContext.constrainWith(TRC->getAvailabilityInfo());
+        if (MostRefined) {
+          *MostRefined = TRC;
+        }
       }
     }
   }