sql: Enable asserts for box/sql.c

Since asserts were always disbled in src/box/sql.c, number
of issues were hidden. This patch enables asserts for mentioned
file by disabling set of NDEBUG in box/src/sqliteInt.h.
Also this patch fixes incorrect assertions about strlen() and
assertion that fieldno should always be less than format->field_count.
This assert is not always hold, e.g. for _schema space where max_id
is stored behind formatted fields.

Closes #3057