bundle7/eid: Fix ipn EID parsing for servicenum == 0
If the service number of a bundle7 EID was set to zero, this resulted in an EID like "ipn:xxx." instead of "ipn:xxx.0", because the allocated length was determined in the wrong manner for the special case of zero.
This also adds tests for the "zero" case and the case of UINT64_MAX.
Signed-off-by: Felix Walter felix.walter@d3tn.com