It's undefined behavior when an application attempts to use a buffer
for which no data has been allocated yet, but this is trivial for us to
check for and produce a non-fatal error.

Bug chromium:736639

Change-Id: I795c22363ada1b5e325d1fb5061a9e7673609879
Reviewed-on: https://swiftshader-review.googlesource.com/10309Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>