readkey: force return from while true

If read_key is feeded by STRG + KEY_LEFT you run into the while(1)
loop and corrupt memory through esc array.
Force return if index gets too high.

Signed-off-by: Jan Weitzel <j.weitzel@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

lib/readkey.c

@@ -67,6 +67,8 @@ int read_key(void)
 				esc[i] = getc();
 				if (esc[i++] == '~')
 					break;
+				if (i == 5)
+					return -1;
 			}
 		}
 		esc[i] = 0;