datapath: Avoid bad array reference for missing group in ODPAT_OUTPUT_GROUP.

datapath: Avoid bad array reference for missing group in ODPAT_OUTPUT_GROUP.

When ODPAT_OUTPUT_GROUP referenced a group that hadn't been created,
output_group() would return a negative number other than -1.
execute_actions() would pass this later to do_output(), which would then
use it as a bogus array index, accessing invalid memory.

This commit fixes it by making it return -1 instead, which the caller
interprets correctly.

Thanks to Justin for leading me to scrutinize this code.