BasePackConnection: Check for expected length of ref advertisement

BasePackConnection: Check for expected length of ref advertisement

When a server sends a ref advertisement using protocol v2 it contains
lines other than ref names and sha1s.  Attempting to get the sha1 out
of such a line using the substring method can result in a SIOOB error
when it doesn't actually contain the sha1 and ref name.

Add a check that the line is of the expected length, and subsequently
that the extracted object id is valid, and if not throw an exception.

Change-Id: Id92fe66ff8b6deb2cf987d81929f8d0602c399f4
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>