Don't crash is dwarf_decode_macro_bytes's 'body' is NULL, even when '!is_define'

Don't crash is dwarf_decode_macro_bytes's 'body' is NULL, even when '!is_define'

Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1715008

On commit 7bede82892a06e6c26989803e70f53697392dcf9 ("Don't crash if
dwarf_decode_macro_bytes's 'body' is NULL"), I was too strict when
checking if 'body' is NULL: the check only comprised the case when
'is_define' is true.  However, the corruption of .debug_macro by
rpmbuild's "debugedit" also affects the case when 'is_define' is
false, i.e., when the macro is being undefined.

This commit improves the check and covers both cases now.  This has
been tested on Fedora 30 with a problematic debuginfo, and I don't see
a segfault anymore.

OK to push?

gdb/ChangeLog:
2019-05-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1708192
	Ref.: https://bugzilla.redhat.com/show_bug.cgi?id=1715008
	* dwarf2read.c (dwarf_decode_macro_bytes): Move check to see if
	'body' is NULL to the outter 'if', protecting the '!is_define'
	situation as well.