gnrc_sixlowpan_iphc_nhc: receiving NHC compressed UDP packets hits assert in IPv6

[miri64]

Fixed by #5281.

See e.g. on iotlab-m3 with examples/gnrc_networking:

m3-12;send fe80::3432:4833:46da:a826 137 abc
1461324728.949442;m3-12;Success: send 2 byte to [fe80::3432:4833:46da:a826]:137)
1461324728.953062;m3-11;> 0x8001f8b
1461324728.953285;m3-11;*** RIOT kernel panic:
1461324728.953543;m3-11;FAILED ASSERTION.
1461324728.953746;m3-11;
1461324728.954449;m3-11;        pid | name                 | state    Q | pri |
stack ( used ) | location
1461324728.957028;m3-11;          1 | idle                 | pending  Q |  15 |
256 (  132 ) | 0x20000220
1461324728.959071;m3-11;          2 | main                 | bl mutex _ |   7 |
1536 (  712 ) | 0x20000320
1461324728.961077;m3-11;          3 | pktdump              | bl rx    _ |   6 |
1536 (  236 ) | 0x20002f48
1461324728.961278;m3-11;          4 | 6lo                  | bl rx    _ |   3 |
1024 (  412 ) | 0x20003558
1461324728.962884;m3-11;          5 | ipv6                 | running  Q |   4 |
1024 (  576 ) | 0x20000e84
1461324728.964820;m3-11;          6 | udp                  | bl rx    _ |   5 |
1024 (  260 ) | 0x20003d60
1461324728.966803;m3-11;          7 | at86rf2xx            | bl rx    _ |   3 |
1024 (  356 ) | 0x20000a04
1461324728.967779;m3-11;            | SUM                  |            |     |
7424 ( 2684 )
1461324728.967976;m3-11;
1461324728.968777;m3-11;*** halted.

(the line of assertion failure is gnrc_ipv6.c:119)

As a workaround until #5281 got merged we deactivated NHC in #5385 for now.

[OlegHahm]

#5281 is merged.