Fix assertion failure in file_tree_read when read operation fails

Initialize *gen to NO_GENERATION at the start of file_tree_read so that
error paths (FILETREE_BADPATH, FILETREE_NOENT, FILETREE_ISDIR) have a
well-defined value. Previously, when file_tree_read failed, gen was
uninitialized, causing the assertion in process_client_read to fire.

The assertion checking gen != NO_GENERATION before the error check was
removed since:
1. On error paths, gen is not used
2. On success paths, the assertion at line 482 already validates gen

src/file_tree.c

@@ -513,6 +513,9 @@ int file_tree_read(FileTree *ft, string path,
     uint64_t off, uint64_t len, uint64_t *gen, uint64_t *chunk_size,
     SHA256 *hashes, int max_hashes, uint64_t *actual_bytes)
 {
+    // Initialize gen to NO_GENERATION so error paths have a well-defined value
+    *gen = NO_GENERATION;
+
     int num_comps;
     string comps[MAX_COMPS];