abspath.c: move declaration of 'len' into inner block and use appropriate type

The 'len' variable was declared at the beginning of the make_absolute_path
function and also in an inner 'if' block which masked the outer declaration.
It is only used in two 'if' blocks, so remove the outer declaration and
make a new declaration inside the other 'if' block that uses 'len'.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>