Do not reuse 'attr' in read_array_type

I was looking at dwarf2/read.c:read_array_type and I thought it would
be nicer if the 'attr' wasn't reused.  This patch changes it to be
redeclared in every 'if' where it is used, making it clear that its
value is local to just the one block.

Approved-By: Simon Marchi <simon.marchi@efficios.com>