sql: move space existence check to VDBE

Before this patch, the existence of space was checked for the
CREATE TABLE or CREATE VIEW statements during the parsing. If the
space already exists, an error has been set and cleanup is
performed. But, if the statement contained 'IF NOT EXIST', the
cleanup was performed, but the error was not set. Meanwhile,
create_foreign_key() assumes that if create_table_def->new_space
is NULL, then we are dealing with ALTER TABLE statement. This in
turn false, since ctd->new_space is nullified also in case of
already existing table. This causes an assertion or a segmentation
fault when creating a foreign key during space creation.

This patch moves this check to VDBE. Parsing now is always
processed till the end as in case space doesn’t exist.

Closes #4196