webpmux: fix type conversion warning
webpmux: fix type conversion warning

Change-Id: I65de23f376570346a9489c2116a0effa60345157

ttyangf/libwebp,Aexyn/libwebp,yohunl/libwebp,redengineer/libwebp,PKRoma/libwebp,Aexyn/libwebp,yohunl/libwebp,Aexyn/libwebp,PKRoma/libwebp,ttyangf/libwebp,PKRoma/libwebp,redengineer/libwebp,yohunl/libwebp,webmproject/libwebp,PKRoma/libwebp,redengineer/libwebp,ttyangf/libwebp,webmproject/libwebp,webmproject/libwebp,redengineer/libwebp,PKRoma/libwebp,PKRoma/libwebp,ttyangf/libwebp,yohunl/libwebp,yohunl/libwebp,Aexyn/libwebp,ttyangf/libwebp,Aexyn/libwebp,webmproject/libwebp,webmproject/libwebp,redengineer/libwebp