[PEi386 linker] Bounds check and null-deref guard

We should resonably be able to expect that we won't exceed the number of
sections if we assume to be dealing with legal object files. We can however
not guarantee that we get some negative values, and while we try to
special case most, we should exclude negative indexing into the sections
array.

We also need to ensure that we do not try to derefences targetSection,
if it is NULL, due to the switch statement.