libdw: check memory access in get_(u|s)leb128

libdw: check memory access in get_(u|s)leb128

__libdw_get_uleb128 and __libdw_get_sleb128 should check if addrp has
already reached the end before unrolling the first step. It is done by
moving __libdw_max_len to the beginning of the function, which can
notice, that addrp is beyond the end. Then we just check the result of
this function.

Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>