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.