[PATCH 15/15] remove variable length array in writeout_multiple_dumpfiles()

Variable length arrays can be problematic. For example with C11 they got
downgraded to an optional feature and thus might not be supported in all
environments. Furthermore they don't allow error checks whether the
allocation succeeded. If it fails this causes undefined behavior. This is
especially problematic when its size is read from a dump as it might have
been corrupted. Thus get rid of variable length arrays and allocate them
on the heap instead.

Signed-off-by: Philipp Rudo <prudo@redhat.com>