fix windows build
fix windows build

broken since:
ad25032 Merge "multi-threaded alpha encoding for lossy"

this produced an error due to an empty VP8TBuffer struct.

Change-Id: I640809d07d20092c1d660e2b59b58a62a12e4371

cysp/libwebp,ya7lelkom/libwebp,Maria1099/webm.libwebp,keane83/BLE_IM,appunite/libwebp,ya7lelkom/libwebp,kalli123/webm.libwebp,kleopatra999/webm.libwebp,altogother/webm.libwebp,ttyangf/libwebp,iniwf/webm.libwebp,comoyo/gonzo-libwebp,Maria1099/webm.libwebp,Maria1099/webm.libwebp,yohunl/libwebp,Distrotech/libwebp,gshORTON/webm.libwebp,jiehu5114/libwebp,jiehu5114/libwebp,PKRoma/libwebp,Acidburn0zzz/webm.libwebp,PKRoma/libwebp,abwiz0086/webm.libwebp,kalli123/webm.libwebp,yohunl/libwebp,Tekserve/libwebp,comoyo/gonzo-libwebp,Maria1099/webm.libwebp,altogother/webm.libwebp,gshORTON/webm.libwebp,Distrotech/libwebp,Acidburn0zzz/webm.libwebp,redengineer/libwebp,iniwf/webm.libwebp,rxl194/libwebp,reimaginemedia/webm.libwebp,comoyo/gonzo-libwebp,Suvarna1488/webm.libwebp,abwiz0086/webm.libwebp,iniwf/webm.libwebp,zofuthan/libwebp,kim42083/webm.libwebp,comoyo/gonzo-libwebp,imazen/libwebp,webmproject/libwebp,cysp/libwebp,Aexyn/libwebp,jiehu5114/libwebp,ttyangf/libwebp,ttyangf/libwebp,Aexyn/libwebp,gshORTON/webm.libwebp,imazen/libwebp,comoyo/gonzo-libwebp,ericmckean/webm.libwebp,zofuthan/libwebp,webmproject/libwebp,imazen/libwebp,ericmckean/webm.libwebp,comoyo/gonzo-libwebp,ericmckean/webm.libwebp,ttyangf/libwebp,Distrotech/libwebp,ya7lelkom/libwebp,yohunl/libwebp,imazen/libwebp,redengineer/libwebp,kleopatra999/webm.libwebp,abwiz0086/webm.libwebp,iniwf/webm.libwebp,Suvarna1488/webm.libwebp,kleopatra999/webm.libwebp,appunite/libwebp,Maria1099/webm.libwebp,Suvarna1488/webm.libwebp,jacklicn/libwebp,ya7lelkom/libwebp,rxl194/libwebp,appunite/libwebp,appunite/libwebp,rxl194/libwebp,PKRoma/libwebp,kalli123/webm.libwebp,comoyo/gonzo-libwebp,Acidburn0zzz/webm.libwebp,matanbs/webm.libwebp,Aexyn/libwebp,kim42083/webm.libwebp,Acidburn0zzz/webm.libwebp,cysp/libwebp,jacklicn/libwebp,jacklicn/libwebp,kleopatra999/webm.libwebp,jacklicn/libwebp,kim42083/webm.libwebp,reimaginemedia/webm.libwebp,gshORTON/webm.libwebp,redengineer/libwebp,ericmckean/webm.libwebp,Tekserve/libwebp,imazen/libwebp,redengineer/libwebp,kim42083/webm.libwebp,Suvarna1488/webm.libwebp,yohunl/libwebp,keane83/BLE_IM,redengineer/libwebp,matanbs/webm.libwebp,kim42083/webm.libwebp,abwiz0086/webm.libwebp,matanbs/webm.libwebp,jacklicn/libwebp,kalli123/webm.libwebp,kleopatra999/webm.libwebp,Aexyn/libwebp,Tekserve/libwebp,cysp/libwebp,reimaginemedia/webm.libwebp,PKRoma/libwebp,ericmckean/webm.libwebp,Acidburn0zzz/webm.libwebp,PKRoma/libwebp,yohunl/libwebp,abwiz0086/webm.libwebp,webmproject/libwebp,altogother/webm.libwebp,keane83/BLE_IM,matanbs/webm.libwebp,Suvarna1488/webm.libwebp,kalli123/webm.libwebp,jiehu5114/libwebp,keane83/BLE_IM,rxl194/libwebp,zofuthan/libwebp,Aexyn/libwebp,gshORTON/webm.libwebp,PKRoma/libwebp,appunite/libwebp,matanbs/webm.libwebp,ya7lelkom/libwebp,webmproject/libwebp,Tekserve/libwebp,webmproject/libwebp,rxl194/libwebp,zofuthan/libwebp,reimaginemedia/webm.libwebp,altogother/webm.libwebp,altogother/webm.libwebp,appunite/libwebp,zofuthan/libwebp,iniwf/webm.libwebp,ttyangf/libwebp,reimaginemedia/webm.libwebp,jiehu5114/libwebp,keane83/BLE_IM