[gdb] Fix cc-with-dwz regression

[gdb] Fix cc-with-dwz regression

I noticed a regression with board cc-with-dwz:
...
FAIL: gdb.cp/m-static.exp: static const int initialized elsewhere
FAIL: gdb.cp/m-static.exp: info variable everywhere
...

The problem started with commit 0494dbecdf "Consolidate partial symtab
dependency reading".

The commit replaces the dwarf2_psymtab::expand_psymtab specific reading of
dependencies, which contains a "dependencies[i]->user == NULL" test, with a
generic partial_symtab::read_dependencies call, which does not test the user
field.

This patch fixes the regression by adding back the test, in the generic
partial_symtab::read_dependencies.

Build and reg-tested on x86_64-linux.

Tested natively, as well as with boards cc-with-dwz and cc-with-dwz-m.

The patch fixes all 33 regressions with cc-with-dwz, and all 2929 regression
with cc-with-dwz-m.

gdb/ChangeLog:

2020-02-21  Tom de Vries  <tdevries@suse.de>

	PR gdb/25534
	* psymtab.c (partial_symtab::read_dependencies): Don't read dependency
	if dependencies[i]->user != NULL.