libelf: Make sure ar_size starts with a digit before calling atol.

libelf: Make sure ar_size starts with a digit before calling atol.

The ar_size field is a 10 character string, not zero terminated, of
decimal digits right padded with spaces.  Make sure it actually starts
with a digit before calling atol on it.  We already make sure it is
zero terminated. Otherwise atol might produce unexpected results.

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