x/tools/gopls: crash in append (invalid string data pointer)

[adonovan]

#!stacks
"crash/crash" && "runtime.memmove" && "strings.(*Builder).WriteString"

Issue created by stacks.

The crashing instruction was a load from the first word of the string data:

move_65through128:
	MOVOU	(SI), X0

indicating that the string is corrupt (data pointer is nil or points to bad memory).

This stack 1Ihixg was reported by telemetry:

• crash/crash
• runtime.throw:+9
• runtime.sigpanic:+33
• runtime.memmove:+161
• strings.(*Builder).WriteString:=108
• strings.Join:+27
• path/filepath.join:+4
• path/filepath.Join:=131
• path/filepath.walk:+19
• path/filepath.walk:+26
• path/filepath.walk:+26
• path/filepath.Walk:+5
• golang.org/x/tools/gopls/internal/filecache.gc:+55
• golang.org/x/tools/gopls/internal/filecache.getCacheDir.func1.gowrap1:+0
• runtime.goexit:+0

golang.org/x/tools/gopls@v0.18.1 go1.23.6 linux/amd64 other,vscode (1)