mongosniff crashes on start

== Begin SRU Template ==
[Impact]

 * mongosniff crashes when it receives messages with no namespace. The result is that a user cannot even press only the enter key without the mongosniff application crashing.
* mongosniff in Xenial and Yakkety is essentially broken currently because of a check in the code that gets the message namespace, but if there is no namespace it crashes.

[Test Case]

 * lxc launch ubuntu-daily:xenial xenial
 * lxc exec xenial bash
 * apt install mongodb
 * mongosniff --source NET lo
 * # Open a 2nd terminal and run the following
 * lxc exec xenial bash
 * mongo
 * # press enter a few times or type 'help'
 * # Observe mongosniff abort, with a core dump, in the first window

[Regression Potential]

* Users currently experiencing this issue would be expecting a SRU fix to come from us as the application is broken in a major way.
* The only work around it would would require rebuilding mongodb from source with the fix to resolve the issue.
* The change was limited to the mongosniff source code only.

[Other Info]

Ubuntu:
* Xenial x64

Packages:
#user@localhost:~$ for i in `dpkg --get-selections|grep -v deinstall|grep mongo|cut -f1`; do echo $i `sudo apt-cache show $i|sed -n -e 's/^.*Version: 1://p'`; done
* mongodb 2.6.10-0ubuntu1
* mongodb-clients 2.6.10-0ubuntu1
* mongodb-server 2.6.10-0ubuntu1

----
Steps to reproduce:

1. start service
2. start sniffer
3. start shell
4. crash sniffer
---

user@localhost:~$ sudo service mongodb start

user@localhost:~$ mongo
MongoDB shell version: 2.6.10
connecting to: test

(...)

user@localhost:~$ sudo mongosniff --source NET lo
sniffing... 27017
127.0.0.1:37522 -->> 127.0.0.1:27017 admin.$cmd 60 bytes id:0 0
 query: { whatsmyuri: 1 } ntoreturn: 1 ntoskip: 0
2016-05-22T01:26:35.545-0400 Assertion failure messageShouldHaveNs() src/mongo/db/dbmessage.cpp 82
2016-05-22T01:26:35.553-0400 0xea35a9 0xe397cf 0xe14772 0x86aca8 0x615c13 0x7f614d369ef6 0x7f614d36e9d1 0x7f614d3731fd 0x616a5a 0x5e57a9 0x7f614c71a830 0x614389
 mongosniff(_ZN5mongo15printStackTraceERSo+0x39) [0xea35a9]
 mongosniff(_ZN5mongo10logContextEPKc+0x21f) [0xe397cf]
 mongosniff(_ZN5mongo12verifyFailedEPKcS1_j+0x142) [0xe14772]
 mongosniff() [0x86aca8]
 mongosniff(_Z10got_packetPhPK11pcap_pkthdrPKh+0x573) [0x615c13]
 /usr/lib/x86_64-linux-gnu/libpcap.so.0.8(+0x5ef6) [0x7f614d369ef6]
 /usr/lib/x86_64-linux-gnu/libpcap.so.0.8(+0xa9d1) [0x7f614d36e9d1]
 /usr/lib/x86_64-linux-gnu/libpcap.so.0.8(pcap_loop+0x5d) [0x7f614d3731fd]
 mongosniff(_Z8toolMainiPPcS0_+0x69a) [0x616a5a]
 mongosniff(main+0x9) [0x5e57a9]
 /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7f614c71a830]
 mongosniff(_start+0x29) [0x614389]
terminate called after throwing an instance of 'mongo::AssertionException'
  what(): assertion src/mongo/db/dbmessage.cpp:82
Aborted (core dumped)
user@localhost:~$