sql: remove unnecessary checks

This patch removes some unnecessary checks that should have been removed
in the 'sql: refactor memory allocation system' patch.

Also, this patch removes the unnecessary check that the return value of
space_by_id() is NULL in sql_check_create(). Return values is not NULL
because sql_check_create() can be called in two cases:
1) space was just created, and space_id is its ID;
1) space_id was obtained directly from the space that existed at the
time the VDBE was created.

Part of tarantool/security#120

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring