bugfix/ota_magic_byte_issue: Fixes OTA magic byte error

On enabling flash encryption, OTA fails with magic byte error in the second chunk
if the http data is split into two or more chunks and first chunk is less than 16.

components/app_update/esp_ota_ops.c

@@ -183,8 +183,7 @@ esp_err_t esp_ota_write(esp_ota_handle_t handle, const void *data, size_t size)
         if (it->handle == handle) {
             // must erase the partition before writing to it
             assert(it->erased_size > 0 && "must erase the partition before writing to it");
-
-            if(it->wrote_size == 0 && size > 0 && data_bytes[0] != 0xE9) {
+            if(it->wrote_size == 0 && it->partial_bytes == 0 && size > 0 && data_bytes[0] != 0xE9) {
                 ESP_LOGE(TAG, "OTA image has invalid magic byte (expected 0xE9, saw 0x%02x", data_bytes[0]);
                 return ESP_ERR_OTA_VALIDATE_FAILED;
             }