Error out immediatly when using if command without args in command list

When using "if" (or while) without args directly on gdb's command line,
you get this:

  (gdb) if
  if/while commands require arguments

When doing the same when entering a command list, you only get an error
when the command is executed, when parse_exp_in_context_1 fails to
evaluate the expression.

  (gdb) define foo
  Type commands for definition of "foo".
  End with a line saying just "end".
  >if
   >end
  >end
  (gdb) foo
  Argument required (expression to compute).

I think it would make more sense to error out when inputting the command
list directly:

  (gdb) define foo
  Type commands for definition of "foo".
  End with a line saying just "end".
  >if
  if/while commands require arguments.

The only required change is to check whether args is an empty string in
build_command_line.

gdb/ChangeLog:

	* cli/cli-script.c (build_command_line): For if/while commands,
	check whether args is empty.

gdb/testsuite/ChangeLog:

	* gdb.base/commands.exp: Call new procedure.
	(define_if_without_arg_test): New procedure.

1 files changed, 2 insertions, 1 deletions

diff --git a/gdb/cli/cli-script.c b/gdb/cli/cli-script.c
index 64b4c2b..1757e4e 100644
--- a/gdb/cli/cli-script.c
+++ b/gdb/cli/cli-script.c
@@ -147,7 +147,8 @@ build_command_line (enum command_control_type type, const char *args)
 {
   struct command_line *cmd;
 
-  if (args == NULL && (type == if_control || type == while_control))
+  if ((args == NULL || *args == '\0')
+      && (type == if_control || type == while_control))
     error (_("if/while commands require arguments."));
   gdb_assert (args != NULL);