lib/decompress: set the compressor name to NULL on error

Without this we end up using the previous name of the compressor in the
loop in unpack_rootfs. For example we get errors like "compression
method gzip not configured" even when we have CONFIG_DECOMPRESS_GZIP
enabled.

Signed-off-by: Aneesh Kumar K.V <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/decompress.c

···
 {
 	const struct compress_format *cf;
 
-	if (len < 2)
+	if (len < 2) {
+		if (name)
+			*name = NULL;
 		return NULL;	/* Need at least this much... */
+	}
 
 	pr_debug("Compressed data magic: %#.2x %#.2x\n", inbuf[0], inbuf[1]);