trap alpha-decoding error
trap alpha-decoding error

Improves robustness to bad bitstream.

Change-Id: I87ca03ccd12d4d690904b35f8dfc2a20a28fb47b

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