Fix nullptr in with_command_1

Running 'with' without arguments crashes GDB.  This fixes it.

gdb/ChangeLog:
2019-08-21  Bogdan Harjoc  <harjoc@gmail.com>

	* cli/cli-cmds.c (with_command_1): Error out if no arguments.

gdb/testsuite/ChangeLog:
2019-08-21  Pedro Alves  <palves@redhat.com>

	* gdb.base/with.exp: Test "with" with no arguments.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2019-08-21  Bogdan Harjoc  <harjoc@gmail.com>
+
+	* cli/cli-cmds.c (with_command_1): Error out if no arguments.
+
 2019-08-21  Christian Biesinger  <cbiesinger@google.com>
 
 	* tui/tui-data.h (tui_gen_win_info): Add an =default

gdb/cli/cli-cmds.c

@@ -217,6 +217,9 @@ void
 with_command_1 (const char *set_cmd_prefix,
 		cmd_list_element *setlist, const char *args, int from_tty)
 {
+  if (args == nullptr)
+    error (_("Missing arguments."));
+
   const char *delim = strstr (args, "--");
   const char *nested_cmd = nullptr;
 

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2019-08-21  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/with.exp: Test "with" with no arguments.
+
 2019-08-21  Tom de Vries  <tdevries@suse.de>
 
 	* gdb.base/gdb-caching-proc.exp: Sort files.

gdb/testsuite/gdb.base/with.exp

@@ -220,6 +220,8 @@ with_test_prefix "run control" {
 
 # Check errors.
 with_test_prefix "errors" {
+    gdb_test "with" "Missing arguments\\."
+
     # Try both an unknown root setting and an unknown prefixed
     # setting.  The errors come from different locations in the
     # sources.