[6.0][RemoteInspection] Ignore MetadataSource records with NULL source pointer.

We'd crash on a call to getKind(). Ignore them instead by returning false from isMetadataSourceReady. If this causes getClosureContextInfo to fail to make forward progress, then it will in turn fail and return nullptr to its caller.

rdar://126866747
(cherry picked from commit 545441e)

Author: mikeash

include/swift/RemoteInspection/ReflectionContext.h

@@ -1989,6 +1989,9 @@ class ReflectionContext
   /// \param Builder Used to obtain offsets of elements known so far.
   bool isMetadataSourceReady(const MetadataSource *MS,
                              const RecordTypeInfoBuilder &Builder) {
+    if (!MS)
+      return false;
+
     switch (MS->getKind()) {
     case MetadataSourceKind::ClosureBinding:
       return true;