fixed possible undefined variable assigment

[EyalBrilling]

On line 5690

n2 = n3

n3 gets a value on line 5657 only if is_zero is false.
On occurences where is_zero is true, n3 keeps its undefined value, potentially effecting n2 and the the rest of the calculations.
Simple fix of defining n3 = 0 - not effecting other calculations if value stays on 0 as it is only used in a '-' operation as the subtrahend.

[NikolajBjorner]

Did you use a tool or do you not have a life?
In any case pretty cool. There are some long standing issues with nlsat, possibly not related to unintialized variables but likely require algorithmic understanding. See github issue tracker for nlsat tagged.

[EyalBrilling]

I used clang static analyzer.
Will look at nlsat,thank you

[nadavrot]

@NikolajBjorner Can you please approve or reject @EyalBrilling's PR?