Malformed VPN record causing gnunet-namestore to crash

Hi all,

I found a crash in the gnunet-namestore command when I am messing with the =
VPN subsystem. Specifically, when I put identity PK into the field of node =
PKEY and put a single-field name into the last parameter. The command will =
crash in a segfault.

Example:

```
~> gnunet-namestore -z marty-on-3090 -a -e "20m" -p -t VPN -n www -V "1 000=
G004CWJENV17NSM444QXKMQNJW136YATCRDPQX27BQDKT1GM1F15TPM http."
Value `1 000G004CWJENV17NSM444QXKMQNJW136YATCRDPQX27BQDKT1GM1F15TPM http.' =
invalid for record type `VPN'
double free or corruption (fasttop)
fish: Job 1, 'gnunet-namestore -z marty-on-30=E2=80=A6' terminated by signa=
l SIGABRT (Abort)
```

Hope this helps the development of GNUnet.

Best,
Martin