The test_ecmult_multi_batch_single is relying on the empty scratch
memory to cause the ecmult algorithm exit early and fail. The tests let
the scalar and point to multiply uninitialized, which is fine since they
will not be used by the algorithm. Unfortunately GCC fails to detect
this situation and will raise -Wmaybe-uninitialized false positive
warnings. Interestingly not all versions are affected, but there is a
handful of bugs with this warning on GCC:
https://gcc.gnu.org/bugzilla/buglist.cgi?quicksearch=maybe-uninitialized
This diffs intializes the operands with random values. It also reduces
the variables sizes because the multiplication applies on a single
point.