[498] Fix memory corruption when shrinking appspace too far
If lazy task swapping is disabled, and appspace is an AMB node, it was possible to map out the page at &7000 and trigger an abort by attempting to shrink application space to a negative size, e.g. by using "*ChangeDyn -RamFSSize" to create a RAM disc larger than the amount of available RAM.
Fix it by sanity checking the DA/AMBNode size within AMBDAHandler before passing on the request to the lower-level growpages/shrinkpages calls (shrinkpages requires the input to be valid and will break if given a negative size).