Choosing single beam structure functions triggers seg fault

[Juergen Reuter]

The following setup

model = "QCD"
process qqgg = u, ubar => g, g
compile
sqrts = 8 TeV
beams = p, ubar => pdf_builtin, none
integrate (qqgg)

triggers a seg fault:

 Process library 'default_lib': loading
| Process library 'default_lib': ... success.
sqrts =  8.000000000000E+03
 Beam data (collision):
   u     (mass = 0.0000000E+00 GeV)
   ubar  (mass = 0.0000000E+00 GeV)
   sqrts = 8.000000000000E+03 GeV
| Beams: u ubar
| Beams: sqrts = 8.000E+03 GeV

Program received signal SIGSEGV: Segmentation fault - invalid memory reference.

Backtrace for this error:
#0  0x184dab3f2
#1  0x184dabbbe

as single beams are not yet supported again ...

[Juergen Reuter]

This now works, if there is still something to do/implement for that case, one can post it under structure function finalization (#500).