mountinfo: GetMountsFromReader() remove redundant switch, fix dropped "optional" fields

[thaJeztah]

This switch was implementing the exact same check as is done in
strings.Join(); https://cs.opensource.google/go/go/+/refs/tags/go1.16.7:src/strings/strings.go;l=421-427

func Join(elems []string, sep string) string {
	switch len(elems) {
	case 0:
		return ""
	case 1:
		return elems[0]
	}

This only difference removing this switch makes is that it will assign
an empty string to p.Optional if no optional fields are present, but
given that an empty string is the default value for p.Optional, this
should make no difference.

This also revealed an off-by-one bug:

Before this change:

• we return an error if sepIdx == 5 (earlier check)
• for sepIdx == 6 we skipped (in this switch)
• for 7, we take field 6 (equivalent to fields[6:7], or fields[6:sepIdx])
• for the "default" (else), we use fields[6:sepIdx-1], which for (e.g.) sepIndex=8
  would do the same as sepIdx=7, so we'd drop one value.

[thaJeztah]

Looks like there's an off-by-one bug here; removing this switch caused it to panic;

--- FAIL: TestMountedBy (0.01s)
panic: runtime error: slice bounds out of range [6:5] [recovered]
	panic: runtime error: slice bounds out of range [6:5]

• we return an error if sepIdx == 5 (earlier check)
• for sepIdx == 6 we skipped (in this switch)
• for 7, we take field 6 (equivalent to fields[6:7], or fields[6:sepIdx])
• for the "default" (else), we use fields[6:sepIdx-1], which for (e.g.) sepIndex=8 would do the same as sepIdx=7, so we'd drop one value

[thaJeztah]

@kolyshkin PTAL

[thaJeztah]

Looking further, I noticed that TestParseMountinfoExtraCases was not checking the Optional field; fixing that test showed that it was indeed broken without this patch, so I included those commits in this PR

[kolyshkin]

LGTM

[thaJeztah]

@AkihiroSuda ptal