Low: libcrmcommon: Check scan_ll() return value in crm_get_msec()

This is redundant since we already check whether the result was set to
its default value. However, Coverity complained that we checked
scan_ll()'s return value in 4 out of 5 places, so it's simplest to just
check it here too.

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