Fix memory leak and use of not initialized memory reported by valgrind

Single step integrators ss2, ss3 and ss4 are affected.

==30840== 40 bytes in 1 blocks are definitely lost in loss record 11 of 16
==30840==    at 0x483BE63: operator new(unsigned long) (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==30840==    by 0xAA3BC9: Solver::Prepare() (solver.cc:674)
==30840==    by 0xA9F2DD: Solver::Run() (solver.cc:2493)
==30840==    by 0x14D7B7A: UnknownInlinedFun (mbdyn.cc:1503)
==30840==    by 0x14D7B7A: mbdyn_program(mbdyn_proc_t&, int, char**, int&) [clone .isra.0] (mbdyn.cc:942)
==30840==    by 0xA61F88: main (mbdyn.cc:1173)