readelf: handle_core_item large right shift triggers undefined behaviour.

readelf: handle_core_item large right shift triggers undefined behaviour.

The problem is this:

  int n = ffs (w);
  w >>= n;

The intent is to shift away up to (and including) the first least
significant bit in w. But w is an unsigned int, so 32 bits. And the
least significant bit could be bit 32 (ffs counts from 1). Unfortunately
a right shift equal to (or larger than) the length in bits of the left
hand operand is undefined behaviour. We expect w to be zero afterwards.
Which would terminate the while loop in the function. But since it is
undefined behaviour anything can happen. In this case, what will actually
happen is that w is unchanged, causing an infinite loop...

gcc -fsanitize=undefined will catch and warn about this when w = 0x80000000

https://bugzilla.redhat.com/show_bug.cgi?id=1259259

Signed-off-by: Mark Wielaard <mjw@redhat.com>