sql: fix sql_vdbe_mem_alloc_region result memory

Function sql_vdbe_mem_alloc_region() that constructs the value
of Vdbe Mem object used to change only type related flags.
However, it is also required to erase other flags (for instance
flags related to allocation policy: static, dynamic etc), since
their combination may be invalid.
In a typical Vdbe scenario, OP_MakeRecord and OP_RowData release
memory with sqlite3VdbeMemRelease() and allocate on region with
sql_vdbe_mem_alloc_region(). An integrity assert based on
sqlite3VdbeCheckMemInvariants() would fire here due to
incompatible combination of flags:
MEM_Static | (MEM_Blob | MEM_Ephem).

Needed for #3850