Some proofs assume that some pointers returned by malloc are not
NULL. This patch modifies those proofs to make these assumptions
explicit with `__CPROVER_assume(pointer != NULL)` for all such
pointers.
Co-authored-by: Mark R. Tuttle <mrtuttle@amazon.com>
Co-authored-by: Aniruddha Kanhere <60444055+AniruddhaKanhere@users.noreply.github.com>