fix obscure crash on unbound variable
fix obscure crash on unbound variable

This can be triggered if someone tries to call a non-ID. The example
reproducer was:

```
if (var = dependency(...)).found()
```

This produced a traceback ending in

```
    raise InvalidArguments(f'Variable "{object_name}" is not callable.')
UnboundLocalError: local variable 'object_name' referenced before assignment
```

After this commit, the error is reported as:

```
ERROR: AssignmentNode is not callable.
```

mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,pexip/meson