Fix size argument in call to memset in MD5Final()

This function isn't used anyway, but the compiler was warning about the incorrect code.

dll/nap/md5.c

@@ -179,7 +179,7 @@ memset(p, 0, count - 8);
     MD5Transform(ctx->buf, (uint32 *) ctx->in);
     byteReverse((unsigned char *) ctx->buf, 4);
     memcpy(digest, ctx->buf, 16);
-    memset(ctx, 0, sizeof(ctx));
+    memset(ctx, 0, sizeof *ctx);
 /* In case it's sensitive */
 }