handle moved mmap after UniqFindTagWrite call

handle moved mmap after UniqFindTagWrite call

A call to UniqFindTagWrite can trigger the need for a bigger mmap, which
is usually done by moving it, but with this move all pointers into it
become invalid (and have to be remapped). The compiler calculates the
pointer before the execution of the call though, so it tries to store
the returned value at the old location, resulting in a segfault.
We solve this by use of a temprorary variable as we did in the other
instances of this problem before.

Closes: #753941