Fix sqlite3_value_text() usage:
- Raise MemoryError if sqlite3_value_text() sets SQLITE_NOMEM
- Let PyUnicode_FromStringAndSize() errors propagate

Quoting the SQLite docs:
"As long as the input parameter is correct, these routines can only fail if an out-of-memory error occurs"


See also:
- bpo-43296
- https://sqlite.org/c3ref/value_blob.html