Fixed a bug allowing the user to delete the shell prompt.

src/shell.c

@@ -25,6 +25,11 @@ void key_pressed(unsigned char* character)
 	}
 	else
 	{
+		if(strlen(commandbuffer) == 0)
+		{
+			//Do not delete the shell prompt, simply return.
+			return;
+		}
 		//Otherwise clear the last character from the command
 		//buffer.
 		commandbuffer[len - 1] = '\0';