fix VLA/continue issue
as reported in
http://lists.nongnu.org/archive/html/tinycc-devel/2015-04/msg00131.html. Note
that this is one of two separate VLA bugs:
A. labels aren't reached by program execution, so the stack pointer is
never saved
B. continue doesn't restore the stack pointer as goto does
This fixes only B. I'm not sure whether the same issue applies to break
as well as continue.
Add a test case, but disable tests #78 and #79 for now as they're not
fully fixed until the issue described in
http://lists.nongnu.org/archive/html/tinycc-devel/2015-04/msg00110.html
is resolved.