input/dahdi.c: Don't simply read beyond end of msgb
input/dahdi.c: Don't simply read beyond end of msgb

Let's first add two bytes to the msgb before writing.  This way we
would assert in case there was no tailroom.  As we just added tailroom
in the previous patch of this series, we are fine

Change-Id: If84b31ea9a3fc7a6c8768918efed2822d1d58427
Closes: OS#4644

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis