[Sema] Fix UB in TypeChecker::computeCaptures()

The function would attempt to walk the body of bodiless members (e.g. stored property accessors). This change simply skips walking the body in such cases.

Author: beccadax

Diff for: lib/Sema/TypeCheckCaptures.cpp

@@ -719,7 +719,8 @@ void TypeChecker::computeCaptures(AnyFunctionRef AFR) {
                           DynamicSelfCaptureLoc,
                           DynamicSelf,
                           AFR);
-  AFR.getBody()->walk(finder);
+  if (AFR.getBody())
+    AFR.getBody()->walk(finder);
 
   unsigned inoutCount = 0;
   for (auto C : Captures) {