Fix IllegalArgumentException when there is no measured width or height.

Should fix Issues:

https://github.com/amlcurran/ShowcaseView/issues/213
https://github.com/amlcurran/ShowcaseView/issues/212
https://github.com/amlcurran/ShowcaseView/issues/200
https://github.com/amlcurran/ShowcaseView/issues/197
https://github.com/amlcurran/ShowcaseView/issues/179