hexen: Fix desync of demo1 on some platforms/compilers

Official release of Hexen's source code relies on unspecified behavior the in order of function's argument evaluation, see ISO-IEC 9899-1999, [6.5.2.2.10]
P_Random() are called in different parameters of P_SpawnMobj() within A_LeafSpawn()
Merge pull request #403 from alexey-lysiuk/hexen_demo1_desync

hexen: Fix desync of demo1 on some platforms/compilers