Ensure we don't send dummy UI frames on BCCH for TC=5
Ensure we don't send dummy UI frames on BCCH for TC=5

When no SI 2bis, nor 2ter, nor 2quater is in use, then the code in
bts_sysinfo_get() will return null, causing the transmission of a dummy
frame (0303012B2B2B2B2B2B2B2B2B2B2B2B2B2B2B2B2B2B2B2B) instead of a
system information message. This is - at least - very odd and might not
be backed by the specification. We should simply send any other system
information message instead of sending a frame that does not have a
valid SI header.

While 030301 might be a valid, empty UI frame on a DCCH, it is not a
valid frame for the BCCH, where the header is structured differently.

In fact, bts_sysinfo_get() should never return NULL and always return a
valid BCCH message.

This bug was found while developing
http://git.osmocom.org/osmo-ttcn3-hacks/tree/sysinfo/Test.ttcn

Change-Id: Ifeaed27d1d7ba9994fb8ce67d660648bcc8efece
Closes: OS#2365

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts