readelf: Use unsigned loop variables in handle_verneed and handle_verdef

readelf: Use unsigned loop variables in handle_verneed and handle_verdef

Prevent signed underflow by changing loop variables to unsigned and
doing count checks before decrementing. This isn't really a bug, but
prevents UB detected by ubsan on fuzzed input. The bad (fuzzed) input
data does get detected anyway.

	* src/readelf.c (handle_verneed): Use unsigned cnt, cnt2.
	(handle_verdef): Likewise.

Signed-off-by: Mark Wielaard <mark@klomp.org>