Fixes an XCode static analyzer warning.

It seems that there could be a situation where the digits part of the z struct is being reallocated and thus the dz pointer is pointing into deallocated memory.

By extracting the digits pointer after the s_qmul call this is prevented. In our project's set of unit tests the functionality is does not break.
Merge pull request #1 from jeroenleenarts/patch-1

Fixes an XCode static analyzer warning.