Low: libcrmcommon: Check invalid time value in pacemakerd API reply

If the XML_ATTR_TSTAMP attribute is present but can't be parsed as an
integer, value_ll gets set to PCMK__PARSE_INT_DEFAULT (-1). This should
never happen, but just in case, we should convert a negative to 0 before
we cast to time_t, an unsigned type.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>