Accidentally, mask which is used to map type of VDBE memory cell into
outer API types was replaced with MEM_TypeMask. But value of the latter
is larger then possible values of VDBE memory cells types. Hence, if it
is applied to memory cell, not only pure types is taken into
consideration, but some additional flags (such as MEM_Zero) as well.
Overall, it results in wrong type calculation for zeroed blobs, for
instance. Lets return back previous mask.

Follow-up #3698
Needed for #3544