Fix 'if (frameRate)' analyze warning

[Jan-E]

Xcode 9 running on macOS High Sierra issues this warning when analyzing with an attached iOS 11.0.1 iPad:

Pods/SDAVAssetExportSession/SDAVAssetExportSession.m:292:17: Converting a pointer value of type 'NSNumber *' to a primitive boolean value; instead, either compare the pointer to nil or call -boolValue

This commit fixes it

[Jan-E]

@rs Please review and merge.

Another solution would be:

if (frameRate != nil)

See the discussion for a comparable PR: https://github.com/NMSSH/NMSSH/pull/235#discussion_r142045590