The function was returning the null pointer for peeks of size zero, which seems like a sensible
thing to do, but is actually pretty easy to get bitten by that if you are extracting a variable
length field which happens to be of zero length and then doing pointer arithmetic on that (which
SymbolFileDWARF does, and ended up crashing in case of empty DW_AT_location).
This changes the function to return a null pointer only when it gets queried for data which is
outside of the range of the extractor, which is more c++-y, as one can still do reasonable things
with pointers to data of size zero (think, end() iterators).
I also add a test and fix some signedness warnings in the existing data extractor tests.