[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 *.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-10  Tom de Vries  <tdevries@suse.de>
+
+	* dwarf2/read.c (process_psymtab_comp_unit_reader): Cast concat NULL
+	sentinel to char *.
+
 2020-02-09  Tom de Vries  <tdevries@suse.de>
 
 	* dwarf2read.c (process_psymtab_comp_unit_reader): Append CU offset to

gdb/dwarf2/read.c

@@ -7260,7 +7260,8 @@ process_psymtab_comp_unit_reader (const struct die_reader_specs *reader,
   else if (strcmp (filename, artificial) == 0)
     {
       debug_filename.reset (concat (artificial, "@",
-				    sect_offset_str (per_cu->sect_off), NULL));
+				    sect_offset_str (per_cu->sect_off),
+				    (char *) NULL));
       filename = debug_filename.get ();
     }
 

gdbsupport/ChangeLog

@@ -1,3 +1,7 @@
+2020-02-10  Tom de Vries  <tdevries@suse.de>
+
+	* environ.c (gdb_environ::set): Cast concat NULL sentinel to char *.
+
 2020-01-24  Christian Biesinger  <cbiesinger@google.com>
 
 	* thread-pool.c (set_thread_name): Add an overload for the NetBSD

gdbsupport/environ.c

@@ -105,7 +105,7 @@ gdb_environ::get (const char *var) const
 void
 gdb_environ::set (const char *var, const char *value)
 {
-  char *fullvar = concat (var, "=", value, NULL);
+  char *fullvar = concat (var, "=", value, (char *) NULL);
 
   /* We have to unset the variable in the vector if it exists.  */
   unset (var, false);