Out of range occurred in some condition
when using `Label` with its `getLetter()` member function.
`Label::recordPlaceholderInfo()` member function is called for
new line (\n) letter in `Label::multilineTextWrap()`,
but the function doesn't set `_lettersInfo[letterIndex].lineIndex`.
But `lineIndex` is used in `Label::updateLabelLetters()`
even if letter is new line.
This change checks `letterInfo.valid` to avoid
accessing `_linesOffsetX[letterInfo.lineIndex]`
for invalid (i.e. `NewLine`) letter.
aacec550c9