fix(gokrb5.config): do not panic on auth_to_local*

Extended:
Also improve error reporting when we encounter incomplete parser
situations. Now, if we are in an unhandled parser situation, we will
return an explicit error instead of panicking on an IndexError.

Previous situation: passing an "auth_to_local" directive caused a Panic
when doing v := strings.TrimSpace(p[1])

config/krb5conf.go

@@ -352,6 +352,10 @@ func (r *Realm) parseLines(name string, lines []string) (err error) {
 			ignore = true
 			err = UnsupportedDirective{"v4 configurations are not supported"}
 		}
+		if strings.Contains(line, "auth_to_local") {
+			ignore = true
+			err = UnsupportedDirective{"auth_to_local* configurations are not supported"}
+		}
 		if strings.Contains(line, "{") {
 			c++
 			if ignore {
@@ -371,7 +375,11 @@ func (r *Realm) parseLines(name string, lines []string) (err error) {
 				continue
 			}
 		}
+		if !strings.Contains(line, "=") {
+			return InvalidErrorf(
+				"gokrb5.config.krb5conf.Realm.parseLines: abnormal parser situation, probably unsupported config directive for realm %s", r.Realm)
 
+		}
 		p := strings.Split(line, "=")
 		key := strings.TrimSpace(strings.ToLower(p[0]))
 		v := strings.TrimSpace(p[1])
@@ -476,6 +484,11 @@ func (d *DomainRealm) parseLines(lines []string) error {
 		if !strings.Contains(line, "=") {
 			return InvalidErrorf("realm line (%s)", line)
 		}
+		if !strings.Contains(line, "=") {
+			return InvalidErrorf(
+				"gokrb5.config.krb5conf.DomainRealm.parseLines: abnormal parser situation, probably unsupported config directive in DomainRealm section %s", strings.Join(lines, "\n"))
+
+		}
 		p := strings.Split(line, "=")
 		domain := strings.TrimSpace(strings.ToLower(p[0]))
 		realm := strings.TrimSpace(p[1])