WebPPictureHasTransparency: add missing pointer check
WebPPictureHasTransparency: add missing pointer check

in the case of use_argb, alpha_offset may be non-zero which would cause
the null pointer check in CheckNonOpaque to pass

fixes a potential crash with invalid width/height set or an integer
sanitizer warning when passing a zeroed picture:
src/enc/picture_csp_enc.c:73:57: runtime error: applying non-zero offset
3 to null pointer

Change-Id: I9d499bba12c65ad5c65d8f9f3c8ee9298ac6081a

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp