memset, memcpy, sizeof consistency fixes

Just as with the OPENSSL_malloc calls, consistently use sizeof(*ptr)
for memset and memcpy.  Remove needless casts for those functions.
For memset, replace alternative forms of zero with 0.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/t1_ext.c

diff --git a/ssl/t1_ext.c b/ssl/t1_ext.c
index 193cae8..f1092ac 100644
--- a/ssl/t1_ext.c
+++ b/ssl/t1_ext.c
@@ -232,7 +232,7 @@
     }
 
     meth = exts->meths + exts->meths_count;
-    memset(meth, 0, sizeof(custom_ext_method));
+    memset(meth, 0, sizeof(*meth));
     meth->parse_cb = parse_cb;
     meth->add_cb = add_cb;
     meth->free_cb = free_cb;