mesa: Fix memory leak in generate_mipmap_compressed.

Fixes Coverity resource leak defect.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/main/mipmap.c

                                       dstWidth, dstHeight, dstDepth,
                                       border, srcImage->InternalFormat,
                                       srcImage->TexFormat)) {
+         free(temp_dst);
          return;
       }