[protobuf-codegen] Missing panic condition

[HeeillWang]

I executed fuzzing, and faces many intentional crashes such like assert statement. Most crashes does not documented with # Panic section, so I'll list up those. Since most of them are quite obvious, here I omits detailed reproduce step. Every cases are triggered from public APIs of protobuf-codegen-3.2.0

Case-1

Thread '<unnamed>' panicked at 'assertion failed: !s.is_empty()', protobuf-codegen-3.2.0/src/gen/rust/ident.rs:13

Triggered from proto_name_to_rs()

Case-2

Thread '<unnamed>' panicked at 'OUT_DIR env var not set: NotPresent', protobuf-codegen-3.2.0/src/codegen/mod.rs:119

Triggered from Codegen::cargo_out_dir()

Case-3

Thread '<unnamed>' panicked at 'root dir in components of rel path: `/`',protobuf-codegen-3.2.0/src/codegen/mod.rs:109

Triggered from Codegen::cargo_out_dir()

Case-4

Thread '<unnamed>' panicked at 'empty rel path: ``', protobuf-codegen-3.2.0/src/codegen/mod.rs:116

Triggered from Codegen::cargo_out_dir()

Case-5

Thread '<unnamed>' panicked at 'parent path in components of rel path: `..`',protobuf-codegen-3.2.0/src/codegen/mod.rs:102

Triggered from Codegen::cargo_out_dir()

Expected patch

• Explicitly mention panic condition on docs with # Panic section