Initialize QuicRstStreamFrame::ietf_error_code in default constructor.
Initialize QuicRstStreamFrame::ietf_error_code in default constructor.

This member currently never gets read without being initialized first, but it's
good practice to always initialize members to avoid test flakiness and UBSAN
failures down the road.

gfe-relnote: n/a, no behavioral change
PiperOrigin-RevId: 307442301
Change-Id: I05ed1480ff4c60a93eb9f24ebacbf8f330b63c06

google/quiche,google/quiche,google/quiche,google/quiche