Create replacement DFD before mutating texture in HDR 4x4 relabel path

The UASTC HDR 4x4 to ASTC HDR 4x4 early exit in
ktxTexture2_TranscodeBasis freed the texture's DFD and relabelled
vkFormat before calling vk2dfd(). If vk2dfd() returned NULL the
function returned an error while leaving the texture with a freed pDfd
and an already-changed vkFormat. Allocate the replacement DFD first and
only then modify the texture, so failure leaves it untouched.

No behavior change on the success path.

Author: SashaRX

lib/src/basis_transcode.cpp

@@ -203,12 +203,16 @@ ktx2transcoderFormat(ktx_transcode_fmt_e ktx_fmt) {
     // Early exit for redundant transcode from UASTC4x4 to ASTC4x4
     if (colorModel   == KHR_DF_MODEL_UASTC_HDR_4x4 &&
         outputFormat == KTX_TTF_ASTC_HDR_4x4_RGBA) {
+        // Create the new DFD before modifying the texture so a failure
+        // leaves the texture unchanged instead of freeing its DFD and
+        // relabelling its vkFormat first.
+        uint32_t* newDfd = vk2dfd(VK_FORMAT_ASTC_4x4_SFLOAT_BLOCK);
+        if (!newDfd)
+            return KTX_OUT_OF_MEMORY;
         // Fix up the current texture
         free(This->pDfd);
+        This->pDfd = newDfd;
         This->vkFormat = VK_FORMAT_ASTC_4x4_SFLOAT_BLOCK;
-        This->pDfd = vk2dfd(VK_FORMAT_ASTC_4x4_SFLOAT_BLOCK);
-        if (!This->pDfd)
-            return KTX_INVALID_VALUE;  // Format is unknown or unsupported.
         return KTX_SUCCESS;
     }