Fix panic in log with empty header

Previously using `l.SetHeader("")` would panic with:

	--- FAIL: TestEmptyHeader (0.00s)
	panic: runtime error: index out of range [-1] [recovered]
		panic: runtime error: index out of range [-1]

	goroutine 18 [running]:
	github.com/labstack/gommon/log.(*Logger).log(0xc00014e990, 0x1, {0x5fb505, 0xd}, {0x0, 0x0, 0x0})
		/home/martin/src/gommon/log/log.go:394 +0x615
	github.com/labstack/gommon/log.(*Logger).Debugf(...)
		/home/martin/src/gommon/log/log.go:158
	github.com/labstack/gommon/log.TestEmptyHeader(0xc000129860?)
		/home/martin/src/gommon/log/log_test.go:128 +0xb1

This adds a check for that. It also checks if there is any content
before writing a space in the "Text header" else branch so you don't end
up with " my message".

log/log.go

@@ -391,7 +391,7 @@ func (l *Logger) log(level Lvl, format string, args ...interface{}) {
 		if err == nil {
 			s := buf.String()
 			i := buf.Len() - 1
-			if s[i] == '}' {
+			if i >= 0 && s[i] == '}' {
 				// JSON header
 				buf.Truncate(i)
 				buf.WriteByte(',')
@@ -404,7 +404,9 @@ func (l *Logger) log(level Lvl, format string, args ...interface{}) {
 				}
 			} else {
 				// Text header
-				buf.WriteByte(' ')
+				if len(s) > 0 {
+					buf.WriteByte(' ')
+				}
 				buf.WriteString(message)
 			}
 			buf.WriteByte('\n')

log/log_test.go

@@ -119,6 +119,16 @@ func TestStringWithQuotes(t *testing.T) {
 	assert.Contains(t, b.String(), `"message":"Content-Type: \"\""`)
 }
 
+func TestEmptyHeader(t *testing.T) {
+	l := New("test")
+	b := new(bytes.Buffer)
+	l.SetOutput(b)
+	l.SetHeader("")
+	l.SetLevel(DEBUG)
+	l.Debugf("captain's log")
+	assert.Contains(t, b.String(), `captain's log`)
+}
+
 func BenchmarkLog(b *testing.B) {
 	l := New("test")
 	l.SetOutput(new(bytes.Buffer))