avoid infinite loop when UrQMD can't generate a collision

Processes/UrQMD/urqmd.f

@@ -337,6 +337,10 @@ cdh        write(*,*)'(W) No collision in event ',event
              print *,'no collision problem in UrQMD'
 c~              stop
            endif
+           if (noc.ge.50000) then
+             call exit(2) ! think of a better way to hand over the error
+                          ! to C++
+           endif
            goto 1
          endif