libelf: Make sure ar_size is terminated when reading ar long names.

The ar_size is given as a fixed size decimal string, right padded with
spaces.  Make sure we read it properly even if there is no terminating
space. Also sanity check len early if we can.

https://sourceware.org/bugzilla/show_bug.cgi?id=24085



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