Index-Out-Of-Bounds panic when using #[serde(skip_serializing_if = "Option::is_none")]

[elad-yosifon]

Code Example:

struct X {
  #[serde(skip_serializing_if = "Option::is_none")]
  y: Option<String>,
  z: Option<i8>
}

Issue Description:

while trying to serialize an instance such as X { y: None, z: Some(0)} using Writer::append_ser(..), it panics with an index-out-of-bounds error pointing to: #/src/ser_schema.rs:376:45

RCA:

• Serde allows for field skipping using the skip_serializing_if directive.
• current implementation assumes that schema field count matchs the number of serializer.serialize(..) calls 100%.
• Use of unsafe random-access on a vec, panics when the assumption breaks.