MD5: Fix clearing of temporary stack memory to use correct length

sizeof of the structure instead of the pointer was supposed to be used
here. Fix this to clear the full structure at the end of MD5Final().

src/crypto/md5-internal.c

@@ -188,7 +188,7 @@ void MD5Final(unsigned char digest[16], struct MD5Context *ctx)
     MD5Transform(ctx->buf, (u32 *) ctx->in);
     byteReverse((unsigned char *) ctx->buf, 4);
     os_memcpy(digest, ctx->buf, 16);
-    os_memset(ctx, 0, sizeof(ctx));	/* In case it's sensitive */
+    os_memset(ctx, 0, sizeof(*ctx));	/* In case it's sensitive */
 }
 
 /* The four core functions - F1 is optimized somewhat */