Added check for negative font size in preferences.

This should allow to automatically recover situations like
https://github.com/arduino/Arduino/issues/6359#issuecomment-324247440

See #6359

arduino-core/src/processing/app/helpers/PreferencesHelper.java

@@ -94,6 +94,8 @@ public abstract class PreferencesHelper {
     try {
       // ParseDouble handle numbers with decimals too
       size = (int) Double.parseDouble(split[2]);
+      if (size < 1) // Do not allow negative or zero size
+        throw new NumberFormatException();
     } catch (NumberFormatException e) {
       // for wrong formatted size pick the default
       size = 12;