Fixed valgrind complaint due to BN_consttime_swap reading uninitialised data.

This is actually ok for this function, but initialised to zero anyway if
PURIFY defined.

This does have the impact of masking any *real* unitialised data reads in bn though.

Patch based on approach suggested by Rich Salz.

PR#3415

(cherry picked from commit 77747e2d9a5573b1dbc15e247ce18c03374c760c)