Silence erroneous uninitialized value warning

The compiler is confused by the circular switch, and thinks that temp
might become uniniitalized. This is clearly false, as it is set at all
switch breaks.

Silence the warning by explicitly initializing it to zero.

main.c

@@ -262,7 +262,7 @@ static void recalc_stops(struct context *ctx, time_t now) {
 }
 
 static void update_temperature(struct context *ctx, time_t now) {
-	int temp, temp_pos;
+	int temp = 0, temp_pos;
 	double time_pos;
 
 	recalc_stops(ctx, now);