Check for invalid length or more than max length for the interpreter, instead

of the validity of the string pointer holding the interpreter.

Submitted by:	sson
Reviewed by:	dim
MFC after:	3 days

usr.sbin/binmiscctl/binmiscctl.c

@@ -363,7 +363,7 @@ add_cmd(__unused int argc, char *argv[], ximgact_binmisc_entry_t *xbe)
 		usage("Error: Missing magic argument");
 	}
 
-	if (!xbe->xbe_interpreter) {
+	if (!strnlen(xbe->xbe_interpreter, IBE_INTERP_LEN_MAX)) {
 		usage("Error: Missing 'interpreter' argument");
 	}