laoder.efi: Fix error in download protcol

The download protocol calls download_data with FileOffset and
BufferLength of 0 first to start the download (no data yet
available). Calls it again with BufferLength == 0 and FileOffset the
size of the download (again, no data). It then starts calling with
BufferLength != 0 and FileOffset == 0 to start the download. The
heuristic I used to detect the start was wrong, so we'd allocate the
buffer twice. Fix that by being more explicit and not using the
heuristic that was bogus.

Fixes: afee781523 ("loader.efi: Recognize new memdisk=<url> and memcd=<url> options")
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D58068

stand/efi/loader/memdisk.c

@@ -50,11 +50,15 @@ download_data(IN VOID *Context, IN VOID *Buffer, IN UINTN BufferLength, IN UINTN
 	dl_state *ctx = Context;
 	decomp_state *dctx = ctx->dctx;
 
+	if (FileOffset == 0 && BufferLength == 0) {
+		printf("Staritng the download\n");
+		return (EFI_SUCCESS);
+	}
+
 	/*
-	 * Make a note of the size when we're hinted about it. But once
-	 * we start the download, ignore the hints.
+	 * Make a note of the size when we're hinted about it.
 	 */
-	if (ctx->size == 0 && BufferLength == 0) {
+	if (BufferLength == 0) {
 		printf("We know we will download %llu bytes\n", ULL(FileOffset));
 		ctx->size = FileOffset;
 		ctx->status = EFI_SUCCESS;