Use cu_header consistently in read_attribute_value

Use cu_header consistently in read_attribute_value

read_attribute_value has a local cu_header variable, but then some
spots in the function use cu->header instead.  It seems better to me
to prefer the local everywhere, so this patch makes this change.

gdb/ChangeLog
2021-03-14  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (read_attribute_value): Use cu_header
	consistently.