skip compound statement with only statements that we cannot represent

skip compound statement with only statements that we cannot represent

01193ea (accept compound statements with zero statements when autodetecting,
Mon Aug 22 13:39:33 2011 +0200) changed pet to accept compound statements
without any statements inside when autodetecting.  However, it also made
pet accept compound statements with one or more statements such that each
of those statements contains a construct that cannot be represented.
This in turn made pet autodetect any compound statement as the first (and
currently only) scop in the input.

Reject compound statements that contain rejected statements and no
valid statements.

Reported-by: Sven van Haastregt <sven.vanhaastregt@arm.com>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>