Dissect trim panics from unicode DELETE (\u007f) char

[montaguethomas]

We hit a panic with parsing logs using dissect with trim when a line had a DELETE (127)(\u007f) character at the end of the line.

• Version: 7.17.0 (affects 7.9 and later)
• Operating System: Linux
• Steps to Reproduce: Test below

panic: runtime error: index out of range [127] with length 127

goroutine 52 [running]:
github.com/elastic/beats/v7/libbeat/processors/dissect.asciiTrimmer.Trim(...)
	/go/src/github.com/elastic/beats/libbeat/processors/dissect/trim.go:66
github.com/elastic/beats/v7/libbeat/processors/dissect.(*Dissector).Dissect(0xc000a0da40, 0xc001517d10, 0xe5, 0x108528980, 0xc0015b1060, 0x0)
	/go/src/github.com/elastic/beats/libbeat/processors/dissect/dissect.go:73 +0x104
github.com/elastic/beats/v7/libbeat/processors/dissect.(*processor).Run(0xc0005b5280, 0xc0015c81c0, 0x107d95b74, 0x1e, 0xc000b8bad0)
	/go/src/github.com/elastic/beats/libbeat/processors/dissect/processor.go:91 +0x3e5

Test to trigger the panic:

func TestTrimmer(t *testing.T) {
	for _, test := range []struct {
		name, cutset    string
		left, right     bool
		input, expected string
	}{
		{
			name:     "trim both with unicode",
			cutset:   " ",
			left:     true,
			right:    true,
			input:    "  hello world! \u007f ",
			expected: "hello world! \u007f",
		},
	} {
		t.Run(test.name, func(t *testing.T) {
			trimmer, err := newTrimmer(test.cutset, test.left, test.right)
			if !assert.NoError(t, err) {
				return
			}
			start, end := trimmer.Trim(test.input, 0, len(test.input))
			output := test.input[start:end]
			assert.Equal(t, test.expected, output)
		})
	}
}

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)