Fix minor leak when we cannot create a mapistore backend context

Valgrind report example:

    457 (118 direct, 339 indirect) bytes in 1 blocks are definitely lost in loss record 9,447 of 11,560
      at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
      by 0x86EE3AC: talloc_vasprintf (in /usr/lib/x86_64-linux-gnu/libtalloc.so.2.1.0)
      by 0x86EED49: talloc_named (in /usr/lib/x86_64-linux-gnu/libtalloc.so.2.1.0)
      by 0x28DBBFDE: mapistore_add_context (mapistore_interface.c:204)
      by 0x288517CF: emsmdbp_object_open_folder (emsmdbp_object.c:439)
      by 0x28851FF6: emsmdbp_object_open_folder_by_fid (emsmdbp_object.c:628)
      by 0x28862D7C: EcDoRpc_RopOpenFolder (oxcfold.c:104)