fix crash when setting font size to 72

[NoahNussbaumer]

This small change fixes an issue where after setting the font sizes to 72 under Edit -> Preferences -> Fonts the application crashes on restart with:

Exception in thread "main" java.lang.IllegalArgumentException: (minimum <= value <= maximum) is false
    at java.desktop/javax.swing.SpinnerNumberModel.<init>(SpinnerNumberModel.java:139)
    at java.desktop/javax.swing.SpinnerNumberModel.<init>(SpinnerNumberModel.java:178)
    at figtree.treeviewer.painters.LabelPainterController.<init>(Unknown Source)
    at figtree.application.FigTreePanel.<init>(Unknown Source)
    at figtree.application.FigTreeFrame.<init>(Unknown Source)
    at figtree.application.FigTreeApplication$3.createDocumentFrame(Unknown Source)
    at jam.framework.MultiDocApplication.createDocumentFrame(Unknown Source)
    at jam.framework.MultiDocApplication.doNew(Unknown Source)
    at figtree.application.FigTreeApplication.main(Unknown Source)