Fix panic in BasicAuthDecode (#14046) (#14048)

* Fix panic in BasicAuthDecode

If the string does not contain ":" that function would run into an
`index out of range [1] with length 1` error. prevent that.

* Update BasicAuthDecode()

Co-authored-by: 6543 <6543@obermui.de>

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: zeripath <art27@cantab.net>

modules/base/tool_test.go

@@ -46,6 +46,12 @@ func TestBasicAuthDecode(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, "foo", user)
 	assert.Equal(t, "bar", pass)
+
+	_, _, err = BasicAuthDecode("aW52YWxpZA==")
+	assert.Error(t, err)
+
+	_, _, err = BasicAuthDecode("invalid")
+	assert.Error(t, err)
 }
 
 func TestBasicAuthEncode(t *testing.T) {