fix a WRONG size argument to a memset() call found by Clang/LLVM

From /dev/humancontroller.

code/qcommon/md5.c

@@ -253,7 +253,7 @@ static void MD5Final(struct MD5Context *ctx, unsigned char *digest)
     
     if (digest!=NULL)
 	    memcpy(digest, ctx->buf, 16);
-    memset(ctx, 0, sizeof(ctx));	/* In case it's sensitive */
+    memset(ctx, 0, sizeof(*ctx));	/* In case it's sensitive */
 }