vinyl: fix compaction crash on disk read error

`vy_slice_stream_next()` clears the return value on failure. This isn't
expected by `vy_write_iterator_merge_step()`, which doesn't update
the source position in the `vy_wirte_iterator::src_heap` in this case.
As a result, an attempt to remove `end_of_key_src` from the heap in
`vy_write_iterator_build_history()` may crash as follows:

```
 # 1  0x572a2ecc21a6 in crash_collect+256
 # 2  0x572a2ecc2be2 in crash_signal_cb+100
 # 3  0x7cfef6645320 in __sigaction+80
 # 4  0x572a2eab16de in tuple_format+16
 # 5  0x572a2eab1a25 in vy_stmt_is_key+24
 # 6  0x572a2eab1be8 in vy_stmt_compare+89
 # 7  0x572a2eab1e37 in vy_entry_compare+74
 # 8  0x572a2eab2913 in heap_less+88
 # 9  0x572a2eab21e3 in vy_source_heap_sift_up+255
 # 10 0x572a2eab20b9 in vy_source_heap_update_node+54
 # 11 0x572a2eab25c1 in vy_source_heap_delete+249
 # 12 0x572a2eab4134 in vy_write_iterator_build_history+1497
 # 13 0x572a2eab4995 in vy_write_iterator_build_read_views+193
 # 14 0x572a2eab4ce6 in vy_write_iterator_next+380
 # 15 0x572a2eadd20b in vy_task_write_run+1132
 # 16 0x572a2eade6cf in vy_task_compaction_execute+124
 # 17 0x572a2eadfa8d in vy_task_f+445
 # 18 0x572a2e9ea143 in fiber_cxx_invoke(int (*)(__va_list_tag*), __va_list_tag*)+34
 # 19 0x572a2eccee7c in fiber_loop+219
 # 20 0x572a2f0aef18 in coro_init+120
```

Normally, a function shouldn't update the return value on failure so
let's fix `vy_slice_stream_next()`.

Closes #10555

NO_DOC=bug fix

(cherry picked from commit f1144c533b6c52c324ffe1cc4fcaeab1f2f6cd9f)