blob: fix segfault on x86_64 with --virtual

blob.ptr might be non-null on empty field in /var/lib/apk/installed
so we compare with blob length.

This fixes a segfault on x86_64.

src/blob.c

@@ -18,7 +18,7 @@ char *apk_blob_cstr(apk_blob_t blob)
 {
 	char *cstr;
 
-	if (blob.ptr == NULL)
+	if (blob.len == 0)
 		return strdup("");
 
 	if (blob.ptr[blob.len-1] == 0)