Zero initialize local variables.

[kevinbackhouse]

Fixes #1717.

#1717 is caused by a local variable not being initialized at canonmn_int.cpp, line 2764.

I decided it would be best to systematically add zero initializers for all similar cases. I used this CodeQL query to find them all:

import cpp
import semmle.code.cpp.dataflow.DataFlow

from LocalVariable v, ArithmeticType t, string path, DataFlow::Node source
where
  v = source.asUninitialized() and
  DataFlow::localFlowStep(source, _) and
  t = v.getType().getUnspecifiedType() and
  path = v.getLocation().getFile().getRelativePath()
select v, t, path

There are 90 remaining results in the xmpsdk sub-directory which I did not fix, because I believe that is 3rd party code that we try to avoid changing.

[hassec]

LGTM 👍