Do not allow protocol messages larger than ~1GB

The PostgreSQL server will reject messages greater than ~1 GB anyway.
However, worse than that is that a message that is larger than 4 GB
could wrap the 32-bit integer message size and be interpreted by the
server as multiple messages. This could allow a malicious client to
inject arbitrary protocol messages.

https://github.com/jackc/pgx/security/advisories/GHSA-mrww-27vc-gghv

pgproto3/function_call.go

@@ -71,10 +71,8 @@ func (dst *FunctionCall) Decode(src []byte) error {
 }
 
 // Encode encodes src into dst. dst will include the 1 byte message type identifier and the 4 byte message length.
-func (src *FunctionCall) Encode(dst []byte) []byte {
-	dst = append(dst, 'F')
-	sp := len(dst)
-	dst = pgio.AppendUint32(dst, 0) // Unknown length, set it at the end
+func (src *FunctionCall) Encode(dst []byte) ([]byte, error) {
+	dst, sp := beginMessage(dst, 'F')
 	dst = pgio.AppendUint32(dst, src.Function)
 	dst = pgio.AppendUint16(dst, uint16(len(src.ArgFormatCodes)))
 	for _, argFormatCode := range src.ArgFormatCodes {
@@ -90,6 +88,5 @@ func (src *FunctionCall) Encode(dst []byte) []byte {
 		}
 	}
 	dst = pgio.AppendUint16(dst, src.ResultFormatCode)
-	pgio.SetInt32(dst[sp:], int32(len(dst[sp:])))
-	return dst
+	return finishMessage(dst, sp)
 }