Don't panic on short input. (#5939)

Fixes #5935

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

pkg/textparse/openmetricsparse.go

@@ -44,7 +44,10 @@ func (l *openMetricsLexer) buf() []byte {
 }
 
 func (l *openMetricsLexer) cur() byte {
-	return l.b[l.i]
+	if l.i < len(l.b) {
+		return l.b[l.i]
+	}
+	return byte(' ')
 }
 
 // next advances the openMetricsLexer to the next character.

pkg/textparse/openmetricsparse_test.go

@@ -419,6 +419,14 @@ func TestOMNullByteHandling(t *testing.T) {
 			input: "a\x00{b=\"ddd\"} 1",
 			err:   "expected value after metric, got \"MNAME\"",
 		},
+		{
+			input: "#",
+			err:   "\"INVALID\" \" \" is not a valid start token",
+		},
+		{
+			input: "# H",
+			err:   "\"INVALID\" \" \" is not a valid start token",
+		},
 	}
 
 	for i, c := range cases {