owl_function_debugmsg: Be sure to match va_start with va_end

Found by Coverity Scan service.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
owl_zephyr_load{subs,loginsubs}: Eliminate useless stat call

Placates Coverity Scan service, which detects a TOCTOU race between
stat and fopen.  This race was in fact hiding an unlikely memory leak.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
owl_variable_bool_get_tostring_default: Remove dead <invalid> case

bool is 0 or 1.  Found by Coverity Scan service.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>