drm/dp: Don't trust drm_dp_downstream_id()

Before we interpret drm_dp_downstream_id() as a string, make sure it is
NULL terminated, even when drm_dp_downtsream_id() fails.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101660
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720174532.23377-2-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_dp_helper.c

@@ -544,7 +544,7 @@ void drm_dp_downstream_debug(struct seq_file *m,
 				 DP_DETAILED_CAP_INFO_AVAILABLE;
 	int clk;
 	int bpc;
-	char id[6];
+	char id[7];
 	int len;
 	uint8_t rev[2];
 	int type = port_cap[0] & DP_DS_PORT_TYPE_MASK;
@@ -583,6 +583,7 @@ void drm_dp_downstream_debug(struct seq_file *m,
 		seq_puts(m, "\t\tType: N/A\n");
 	}
 
+	memset(id, 0, sizeof(id));
 	drm_dp_downstream_id(aux, id);
 	seq_printf(m, "\t\tID: %s\n", id);