Fix low memory crashes downstream of failure of mmap inside chunk init.

Fail -[FICImageTableChunk initWithFileDescriptor:index:length:] when unable to memory map the backing file for a chunk. The existing code would return a chunk with NULL _bytes. This lead to subsequent crashes when indirecting through _bytes for image operations. i.e. `-[FICImageTableEntry _metaData]` and downstream code using the `[self _metadata]->xxx = yyy;` pattern.

By simply returning a nil chunk, the code is much less likely to crash.