Quiet warning C2220

Quiet warning C2220

This warning reports "potentially uninitialized local variable
`fieldOffset` used".  The use is guarded by a check on `fieldHnd` being
non-null, and `fieldOffset` gets initialized properly on all paths that
set `fieldHnd` to something other than null, so the fix is simply to
initialize `fieldOffset` to 0 at its declaration so the compiler won't
complain.

Resolves #11800.