Static analysis warnings in MGLComputedShapeSource.mm

[akitchen]

The changes in #9983 introduce three static analysis warnings in MGLComputedShapeSource.mm.

In -[MGLComputedShapeSource initWithIdentifier:options:] we are referencing self and an instance variable before self is initialized. This should be simple enough to address with a refactoring to use a local variable for requestQueue, setting _requestQueue = requestQueue once self is initialized.

The other two warnings are less obvious to me (1st function call argument is an uninitialized value (within a call to '~variant')).

/cc @asheemmamoowala