helper: command: assert on command_print() with no command_invocation

Commit 7f260f5009 ("helper/command: Handle Tcl return values
consistently") allows calling command_print with the first
parameter set to NULL.
This should not be allowed.
I cannot identify any part of OpenOCD that calls a command that in
turn calls command_print() with NULL command_invocation.
Adding
	__attribute__((nonnull (1, 2)))
to the prototype of the functions does not trigger any positive
with GCC and scan-build.

Add an assert to detect such corner cases.
This change is kept small and self contained to allow reverting it
easily if a real use case is identified.

Change-Id: I8aa6e5c0c341e818c0327eaa0d2bd5b6304c93b8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/9091
Tested-by: jenkins

src/helper/command.c

@@ -348,13 +348,7 @@ void command_output_text(struct command_context *context, const char *data)
 static void command_vprint(struct command_invocation *cmd,
 		va_list ap, const char *format, bool add_lf)
 {
-	/*
-	 * FIXME: Why this check on !cmd ?
-	 * Commit 7f260f5009a7 that introduces it, does not explain why!
-	 * Was author not confident on the code change?
-	 */
-	if (!cmd)
-		return;
+	assert(cmd);
 
 	// Quit on previous allocation error
 	if (cmd->output == CMD_PRINT_OOM)