[vm] ffi_use_sites: Adds TreeNode.location null check.
[vm] ffi_use_sites: Adds TreeNode.location null check.

TreeNode.location may return null "if the node is orphaned".
This happens for instance in cases where nodes can't use a
fileOffset due to patch files (see #31579).

This will in turn obscure other errors as the error reporting
code will crash trying to access members on null.

TEST=existing.

Bug: https://github.com/dart-lang/sdk/issues/31579
Change-Id: I85b720ecf2cb09e46cbd1296671be523567d5a83
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/202920
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Commit-Queue: Clement Skau <495593248b6158163719b90dde8357b589c8e5d0@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk