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vinyl: handle error loading statement from disk during key lookup

`vy_page_stmt()` may fail (return NULL) if:
 - the statement is corrupted;
 - memory allocation for the statement fails;
 - the statement size exceeds `box.cfg.vinyl_max_tuple_size`.

If this happens `vy_page_find_key()` won't return an error. Instead,
it'll either point the caller to a wrong statement or claim that there's
no statement matching the key in this page. This may result in invalid
index selection results and, later on, a crash caused by inconsistencies
in the tuple cache. The issue was introduced by commit ac8ce0233a00
("vinyl: factor out function to lookup key in page").

All of the three cases are actually very unlikely to happen in
production:
 - If a statement stored in a run file is corrupted, we'll probably fail
   to load the whole page due to failed checksums and never even get to
   `vy_page_stmt()`.
 - Statements are allocated with `malloc()`, which doesn't normally
   fail (instead the whole process would be terminated by OOM) .
 - Users don't tend to lower the tuple size limit after restart.

Still, let's fix the issue by implementing proper error handling for
`vy_page_find_key()`.

Closes #10512

NO_DOC=bug fix

(cherry picked from commit 9dbaa6a9b)

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