When we have a GError* variable on the stack, we usually want to pass
it on to function that can fail. In that case, the variable MUST be
initialized to NULL. This is an easy mistake to make.
Note that this check still can have lots of false positives, for
example, if you have a struct with an GError field. In that case, you
would need to ensure that the entire struct is initialized. Ignore the
warning then.
Also, the check misses if you declare multiple variables on one line.
But that is already discouraged by our style.
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