In the generated code of previous versions, each php field is given an
initial value. In c extension, it was assumed that the field order in
the generated code is consistent with upb fields order, so that the
correct initial value can be bound to the correct upb field. However,
this may not be true. The order of fields in generated code is decided
by proto compiler, while the order of upb fields is decided by the hash
function used in c extension.
This PR fixes the issue by reset the initial value at runtime.