Ensure matcher values are present when parsing matchers from strings

Fixes #2936

Signed-off-by: gotjosh <josue.abreu@gmail.com>

pkg/labels/parse.go

@@ -120,8 +120,12 @@ func ParseMatcher(s string) (_ *Matcher, err error) {
 		return nil, errors.Errorf("bad matcher format: %s", s)
 	}
 
+	rawValue := ms[3]
+	if len(rawValue) == 0 {
+		return nil, errors.Errorf("matcher value is not present: %s", s)
+	}
+
 	var (
-		rawValue            = ms[3]
 		value               strings.Builder
 		escaped             bool
 		expectTrailingQuote bool

pkg/labels/parse_test.go

@@ -189,6 +189,10 @@ func TestMatchers(t *testing.T) {
 				return append(ms, m, m2)
 			}(),
 		},
+		{
+			input: `job=`,
+			err:   `matcher value is not present: job=`,
+		},
 		{
 			input: `job="value`,
 			err:   `matcher value contains unescaped double quote: "value`,