fix issue where prompt height could be garbage value
fix issue where prompt height could be garbage value

jlukanta/mixpanel-iphone,jlukanta/mixpanel-iphone,senselabs/mixpanel-iphone,senselabs/mixpanel-iphone,senselabs/mixpanel-iphone,senselabs/mixpanel-iphone,senselabs/mixpanel-iphone,bloom/mixpanel-osx-unofficial,senselabs/mixpanel-iphone,jlukanta/mixpanel-iphone