Fix crash when fileURLWithPath: paragram = nil

[wenluma]

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Pull Request 说明 Pull Request Description

这个 Pull Request 解决了如下问题/加入了如下功能：···

This pull request fixes / reffers to the following issues: ...

[wenluma]

Fix crash when fileURLWithPath: paragram = nil

[skyline75489]

I'd prefer NSParameterAssert

[wenluma]

tempPath = [[self incompleteDownloadTempCacheFolder] stringByAppendingPathComponent:md5URLString];
tempPath maybe nil, then crash

[skyline75489]

When would tempPath be nil?

[wenluma]

if(![fileManager createDirectoryAtPath:cacheFolder withIntermediateDirectories:YES attributes:nil error:&error])
'incompleteDownloadTempCacheFolder' maybe nil