自定义cell，快速滚动页面时候，可能出现崩溃

[dippac]

2017-03-15 17:52:08.625095 FSCalendarExample[3032:200482] *** Terminating app due to uncaught exception 'CALayerInvalidGeometry', reason: 'CALayer position contains NaN: [207 nan]'
*** First throw call stack:
(0x182d851b8 0x1817bc55c 0x182d85100 0x1860dd19c 0x1860dd310 0x1860dd8a8 0x188c34174 0x188c58440 0x188ce9c14 0x1000d850c 0x1000d4370 0x1000ccb6c 0x1000cba18 0x1000aac3c 0x189202f80 0x189206688 0x188dcd73c 0x188c6c0f0 0x1891f6680 0x1891f61e0 0x1891f549c 0x188c6a30c 0x188c3ada0 0x18942475c 0x18941e130 0x182d32b5c 0x182d324a4 0x182d300a4 0x182c5e2b8 0x184712198 0x188ca57fc 0x188ca0534 0x1000959a0 0x181c415b8)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb) 重现方式：demo中，将DIY改为全屏，手指左右滑动，demo发生崩溃

• 安装方式. `如手动安装。
• 能够重现此问题的完整步骤。
• 设备型号及版本。 iPhone6s plus ios10.2
• 使用的Xcode版本。 Xcode8.1
• FSCalendar版本. FSCalendar 2.7.4

[WenchaoD]

Fixed.