vis_pipe: correctly return non-zero exit status

according to POSIX wait(3p) the return status needs to be checked
and the macro WEXITSTATUS(stat_val) should be used to get the actual
return value on a normal exit. POSIX doesn't specify the value of
stat_val on abnormal exit and thus vis_pipe() should just return
-1 as it does for other errors

closes #1130: vis:pipe returns wrong exit status (when non-zero)

Thanks @pippi1otta for the report and suggestion.

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