vshInit: Don't leak @histsize_env

vshInit: Don't leak @histsize_env

Caller is responsible for freeing the result of virStringJoin()
when no longer needed:

==10701== 1 bytes in 1 blocks are definitely lost in loss record 1 of 806
==10701==    at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==10701==    by 0xAADB679: strdup (in /lib64/libc-2.20.so)
==10701==    by 0x4F18655: virStrdup (virstring.c:726)
==10701==    by 0x4F175AF: virStringJoin (virstring.c:165)
==10701==    by 0x131D4D: vshReadlineInit (vsh.c:2572)
==10701==    by 0x1322DF: vshInit (vsh.c:2736)
==10701==    by 0x1347C1: main (virsh.c:907)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>