vk::allocate() zero-initializes its memory, which was nulling the
VkSpecializationInfo::pMapEntries field in case the mapEntryCount is 0.

We'll be changing vk::allocate() to not zero-initialize, to catch cases
where applications might incorrectly be relying on it, with
MemorySanitizer. So we have to also not be relying on it.

Bug: b/140991626
Change-Id: I5917c902c0af27985556340dfc1dbae79010a125
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54468
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>