[gdb] Fix -Wstrict-null-sentinel warnings

When passed in CXXFLAGS, -Wstrict-null-sentinel triggers twice in a
gdb/gdbserver build.

Fix the two occurrences.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-02-10  Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c (process_psymtab_comp_unit_reader): Cast concat NULL
	sentinel to char *.

gdbsupport/ChangeLog:

2020-02-10  Tom de Vries  <tdevries@suse.de>

	* environ.c (gdb_environ::set): Cast concat NULL sentinel to char *.