Add NULL Pointer check for MACH0_(obj_t) pointer

[7erryX]

CVE-2022-1649

void MACH0_(iterate_chained_fixups) in libr/bin/format/mach0/mach0.c once caused NULL Pointer Dereference by visiting a MACH0_(obj_t) pointer directly.

ut16 page_start = obj->chained_starts[i]->page_start[page_idx];

It was fixed in Commit a5aafb9

+				if (!bin->chained_starts[i]->page_start) {
+					break;
+				}
				ut16 page_start = bin->chained_starts[i]->page_start[page_idx];

Similar Issue

rebase_buffer_fixup in libr/bin/p/bin_xnu_kernelcache.c contains a similar logic

	struct MACH0_(obj_t) *obj = kobj->mach0;
       ...
		ut64 page_size = obj->chained_starts[i]->page_size;

Would it make sense to fix it by adding the pointer check in the same way Commit a5aafb9 did?

Thank you for spending time reading this issue. Apologies if I missed anything.

[trufae]

It wont hurt to add an extra null check there but without having a reproducer or looking in detail i cant confirm it. Would you like to submit a pr with the proposed patch?

[trufae]

Also i think theres a div by zero at rebase_buffer_fixup that maybe worth checking too