Debugger: DwarfImageDebugInfo - fix potential crash.

If the subroutine address in question required us to in a different
image, that lookup would overwrite our pointer to the starting image.
If said lookup then also failed, a crash would occur when performing
operations to look up the next return value, since the image pointer
would then be NULL.
Debugger: Implement ArchitectureX8664::GetReturnAddressLocation().
Debugger: Fix some handling in DisassemblerX8664.

Take into account the operand and offset size as needed, otherwise
we'd potentially use the wrong value for our calculations.

Gets return values somewhat working on x86-64, though in some cases
looking up a PLT slot's destination fails, for as yet undetermined
reasons.