autocomplete: prevent slice bounds panic

[shwstppr]

Fixes #195

[github-actions]

✅ Build complete for PR #196.

🔗 Download the cmk binaries (expires on October 17, 2025)

[hsato03]

LGTM.

I was able to reproduce the issue in the autocomplete of the id field in the listAccounts API.

Apache CloudStack 🐵 CloudMonkey 6.5.0
Report issues: https://github.com/apache/cloudstack-cloudmonkey/issues

panic: runtime error: slice bounds out of range [:30] with capacity 8

goroutine 35 [running]:
github.com/chzyer/readline.(*opCompleter).truncateBufferAfterLastEqual(0xc000191340, {0xc0015e6560, 0x7, 0x8})
	/home/shwstppr/lab/shapeblue/cloudstack-cloudmonkey/vendor/github.com/chzyer/readline/complete.go:54 +0x1e9
github.com/chzyer/readline.(*opCompleter).writeRunes(0xc000191340, {0xc0015e6540, 0x10000c0016ebc28?, 0x10})
	/home/shwstppr/lab/shapeblue/cloudstack-cloudmonkey/vendor/github.com/chzyer/readline/complete.go:70 +0x8d
github.com/chzyer/readline.(*opCompleter).OnComplete(0xc000191340)
	/home/shwstppr/lab/shapeblue/cloudstack-cloudmonkey/vendor/github.com/chzyer/readline/complete.go:125 +0x1b2
github.com/chzyer/readline.(*Operation).ioloop(0xc000191260)
	/home/shwstppr/lab/shapeblue/cloudstack-cloudmonkey/vendor/github.com/chzyer/readline/operation.go:180 +0xd0f
created by github.com/chzyer/readline.NewOperation in goroutine 1
	/home/shwstppr/lab/shapeblue/cloudstack-cloudmonkey/vendor/github.com/chzyer/readline/operation.go:88 +0x2c6

When attempting to reproduce the problem with the cmk binaries from PR, the panic no longer occurs.