Gazelle buf extension crashes with nil pointer dereference when proto language is not loaded

[ctcjab]

Description

The buf Gazelle extension crashes with a nil pointer dereference when the proto Gazelle language is not also enabled. This happens because loadConfig in gazelle/buf/config.go:117 calls proto.GetProtoConfig(gazelleConfig) without checking for nil.

Steps to Reproduce

1. Use aspect_gazelle_prebuilt (or any Gazelle binary with the buf extension compiled in)
2. Enable buf as a language but not proto:

   aspect_gazelle(
       name = "gazelle",
       languages = [
           "buf",
           "python",
           "starlark",
       ],
   )

3. Have a buf.yaml in the repo
4. Run bazelisk run gazelle

Expected Behavior

Gazelle should either:

• Handle the case gracefully (skip proto-specific config when the proto language isn't loaded), or
• Emit a clear error message saying that buf requires proto to also be enabled

Actual Behavior

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0x24fcdde]

goroutine 1 [running]:
github.com/bufbuild/rules_buf/gazelle/buf.loadConfig(0xf84b825a270, {0xf84b87a74d0, 0x22}, 0xf84b8bda8c0)
	external/rules_buf+/gazelle/buf/config.go:117 +0x47e
github.com/bufbuild/rules_buf/gazelle/buf.(*bufLang).Configure(0x2f5f40?, 0xf84b825a270, {0xf84b87a74d0?, 0x22?}, 0xf84b8bda8c0?)
	external/rules_buf+/gazelle/buf/config.go:65 +0x2d

Root Cause

In gazelle/buf/config.go line 117:

protoConfig := proto.GetProtoConfig(gazelleConfig)
stripImportPrefix := "/" + packageRelativePath
if protoConfig.StripImportPrefix == "" {

proto.GetProtoConfig(gazelleConfig) returns nil when the proto Gazelle language isn't registered, and the subsequent field access panics.

Environment

• Bazel 9.1.0
• rules_buf 0.5.4
• aspect_gazelle_prebuilt 0.0.11
• buf.yaml v2 format