fuzz_imagedecoder: ignore allocation failures

Bug: 142252770
Bug: 169137236
Bug: 169139756
Test: this

Image decoders may attempt to allocate a large amount of memory
(especially if the encoded image is large). This doesn't necessarily
mean there is a bug. Set allocator_may_return_null = 1 for hwasan so the
fuzzer can continue running.

Change-Id: Ib750a142e28d01144036409786e631dfb783283f