x/tools/gopls: SEGV in json.Marshal

[adonovan]

#!stacks
"sigpanic" &&  "abi.Name.IsExported" && "json.Marshal"

Issue created by stacks.

More memory corruption, no doubt.

This stack Kmi83g was reported by telemetry:

• crash/crash
• runtime.throw:+9,+0x33
• runtime.sigpanic:+33,+0x213
• internal/abi.Name.IsExported:=610,+0x58
• reflect.Value.Field:+14,+0x48
• encoding/json.structEncoder.encode:+15,+0x2d3
• encoding/json.structEncoder.encode-fm:+0,+0x63
• encoding/json.arrayEncoder.encode:+7,+0xbf
• encoding/json.arrayEncoder.encode-fm:+0,+0x4b
• encoding/json.sliceEncoder.encode:+20,+0x25b
• encoding/json.sliceEncoder.encode-fm:+0,+0x4b
• encoding/json.(*encodeState).reflectValue:+1,+0x6f
• encoding/json.(*encodeState).marshal:+10,+0xa7
• encoding/json.Marshal:+4,+0x83
• golang.org/x/tools/internal/jsonrpc2.marshalToRaw:=233,+0x43
• golang.org/x/tools/internal/jsonrpc2.NewResponse:+1,+0x78
• golang.org/x/tools/internal/jsonrpc2.(*conn).run.(*conn).replier.func1:+10,+0xab
• golang.org/x/tools/gopls/internal/protocol.Handlers.CancelHandler.func3.1:+6,+0xa7
• golang.org/x/tools/gopls/internal/protocol.Handlers.CancelHandler.CancelHandler.func4.1:+4,+0x9f

golang.org/x/tools/gopls@v0.20.0 go1.25.1 darwin/arm64 other,vscode (1)