facts: handle overlapping prefix/suffix

We have a generated file which starts with the comment:

```
// Code generated DO NOT EDIT.
```

This causes staticcheck to panic, because we have an overlapping prefix and
suffix when checking for these magic strings, i.e.:

```
"// Code generated "
                 " DO NOT EDIT."
```

Trying to take a slice where end is before start subsequently panics.

Add an explicit check to avoid the out of bounds access.

Closes: gh-716 [via git-merge-pr]
(cherry picked from commit dce81a6)

Author: kjgorman

facts/generated.go

@@ -46,6 +46,10 @@ func isGenerated(path string) (Generator, bool) {
 		s = bytes.TrimSuffix(s, crnl)
 		s = bytes.TrimSuffix(s, nl)
 		if bytes.HasPrefix(s, prefix) && bytes.HasSuffix(s, suffix) {
+			if len(s)-len(suffix) < len(prefix) {
+				return Unknown, true
+			}
+
 			text := string(s[len(prefix) : len(s)-len(suffix)])
 			switch text {
 			case "by goyacc.":