output: fix wlr_output_set_gamma() with zero size

This is documented to reset the gamma LUT, but we don't handle this
properly.

While at it, make sure we leave wlr_output.pending in a good state
on allocation failure.

types/output/output.c

@@ -925,18 +925,22 @@ void wlr_output_send_present(struct wlr_output *output,
 
 void wlr_output_set_gamma(struct wlr_output *output, size_t size,
 		const uint16_t *r, const uint16_t *g, const uint16_t *b) {
+	uint16_t *gamma_lut = NULL;
+	if (size > 0) {
+		gamma_lut = malloc(3 * size * sizeof(uint16_t));
+		if (gamma_lut == NULL) {
+			wlr_log_errno(WLR_ERROR, "Allocation failed");
+			return;
+		}
+		memcpy(gamma_lut, r, size * sizeof(uint16_t));
+		memcpy(gamma_lut + size, g, size * sizeof(uint16_t));
+		memcpy(gamma_lut + 2 * size, b, size * sizeof(uint16_t));
+	}
+
 	output_state_clear_gamma_lut(&output->pending);
 
 	output->pending.gamma_lut_size = size;
-	output->pending.gamma_lut = malloc(3 * size * sizeof(uint16_t));
+	output->pending.gamma_lut = gamma_lut;
-	if (output->pending.gamma_lut == NULL) {
-		wlr_log_errno(WLR_ERROR, "Allocation failed");
-		return;
-	}
-	memcpy(output->pending.gamma_lut, r, size * sizeof(uint16_t));
-	memcpy(output->pending.gamma_lut + size, g, size * sizeof(uint16_t));
-	memcpy(output->pending.gamma_lut + 2 * size, b, size * sizeof(uint16_t));
-
 	output->pending.committed |= WLR_OUTPUT_STATE_GAMMA_LUT;
 }