crypto: consistently use size_t as type for length parameters

size_t is the correct type to use for sizes.

doc/APIchanges

@@ -13,6 +13,10 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-xx-xx - xxxxxxx
+  Change av_sha_update() and av_md5_sum()/av_md5_update() length
+  parameter type to size_t at next major bump.
+
 2016-xx-xx - xxxxxxx - lavc 57.29.0 - avcodec.h
   Add AV_PKT_DATA_SPHERICAL packet side data to export AVSphericalMapping
   information from containers.