It is an invalid operation to map an already mapped buffer.

Change-Id: Iba1c2389e5ef9c3114a2415279406ac7f08a0ed6
Reviewed-on: https://swiftshader-review.googlesource.com/13750Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>