ignore empty clusters when getting first block
ignore empty clusters when getting first block

Change-Id: I9278efcacc3ea5dcfa584cb0937e39a4004e6947

zofuthan/libwebm,Acidburn0zzz/webm.libwebm,matanbs/webm.libwebm,Suvarna1488/webm.libwebm,kleopatra999/webm.libwebm,kalli123/webm.libwebm,reimaginemedia/webm.libwebm,felipebetancur/libwebm,webmproject/libwebm,abwiz0086/webm.libwebm,iniwf/webm.libwebm,gshORTON/webm.libwebm,webmproject/libwebm,webmproject/libwebm,Maria1099/webm.libwebm,pcwalton/libwebm,felipebetancur/libwebm,zofuthan/libwebm,jdm/libwebm,kim42083/webm.libwebm,ericmckean/webm.libwebm