SPDY: fixed off_t/size_t type conversions on 32 bits platforms. Parameters of ngx_http_spdy_filter_get_shadow() are changed from size_t to off_t since the last call of the function may get size and offset from the rest of a file buffer. This fixes possible data loss rightfully complained by MSVC on 32 bits systems where off_t is 8 bytes long while size_t is only 4 bytes. The other two type casts are needed just to suppress warnings about possible data loss also complained by MSVC but false positive in these cases.