sql: properly check fieldno in sql.c

Tuples that have been inserted into system spaces after the _space
definition has been inserted into _space have the same format as the
space into which they were inserted. However, previously inserted tuples
have an incomplete tuple format with parts missing. One piece that is
missing information is the JSON token, which is used to determine the
number of fields.

The tarantoolsqlIdxKeyCompare() function contains the correct checks for
the case when the number of fields from the format is equal to or less
than the fieldno field, but uses tuple_format_field() before this check,
resulting in an assertion.

This patch forces tarantoolsqlIdxKeyCompare() to call
tuple_format_field() only after checking that fieldno is less than
field_count.

Closes #8418

NO_DOC=Bugfix in debug.

(cherry picked from commit 2252fa7b)