UBSAN pointed out that memcpy cannot have nullptr dest or source,
which I've also fixed; however, this also made me realize a latent bug:
the dest and source args needed to be swapped. It seems this bug hasn't
bit us yet because the normal use-case is to resize the bitvector, and
then call 'set' to set all bits to 0 or 1. In any case, still worth
fixing.

Bug: b/171581990
Change-Id: Iaaa7e019b53ae661f49c97195f2d144b5715f164
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49528
Presubmit-Ready: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>