[mucurses] Attempt to fix use of uninitialised buffer with strcat()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/mucurses/slk.c

 
 	assert ( slks->max_label_len <= SLK_MAX_LABEL_LEN );
 	space_ch = ' ';
+	memset ( str, 0, sizeof ( str ) );
 
 	// protect against gaps in the soft label keys array
 	if ( ! sl.label[0] ) {