Zero encoded_remaining when a string field doesn't fit, so that we don't leave partial data in the buffer (all decoders should ignore it anyway) and to be sure that we don't try to put any subsequent operands in either (there shouldn't be enough space).

PiperOrigin-RevId: 490143656
Change-Id: I4d743dd9214013fbd151478ef662d50affd5ff7a