Fixed memory leaks pointed out by Clang

Fixed potential memory leaks pointed out by the Clang static analyzer:
* **DTMultiExpressionPatch#compileSourceOfType:**  
    `QCMathematicalExpression *expr` is released properly… unless the `error` condition is hit.  
    Now `release`s before `continue`ing in the error case.
* **DTSampleHoldPatch#setState:**  
    The result of NSKeyedUnarchiver#unarchiveObjectWithData: is double-retained, once explicitly and then again by the retaining `priorValue` property.  
    Removed the explicit retain.