qapi: Fix missing 'if' checks in struct, union, alternate 'data'

qapi: Fix missing 'if' checks in struct, union, alternate 'data'

Commit 87adbbffd4..3e270dcacc "qapi: Add 'if' to (implicit
struct|union|alternate) members" (v4.0.0) neglected test coverage, and
promptly failed to check the conditions.  Review fail.

Recent commit "tests/qapi-schema: Demonstrate insufficient 'if'
checking" added test coverage, demonstrating the bug.  Fix it by add
the missing check_if().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-13-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>