experimental string processing crash on very long variable name

[JelleZijlstra]

On main with --experimental-string-processing:

% cat espcrash.py 
xxxxxx_xxx_xxxx_xx_xxxxx_xxxxxxxx_xxxxxxxx_xxxxxxxxxx_xxxx_xxxx_xxxxx = xxxx.xxxxxx.xxxxxxxxx.xxxxxxxxxxxxxxxxxxxx(
    xx_xxxxxx={
        "x3_xxxxxxxx": f"{x3_xxxxx_xxxxxxxx_xxxxxxxx_xxxxxxxxxx_xxxxxxxx_xxxxxx}_xxxxxxx",
    },
)
% black --experimental-string-processing espcrash.py         
error: cannot format espcrash.py: INTERNAL ERROR: hand-made leaf without parent: Leaf(STRING, '"x3_xxxxxxxx"')
Oh no! 💥 💔 💥
1 file failed to reformat.

cc @bbugyi200

[bbugyi200]

Thanks for tagging me on this. This issue should be fixed by the following PR: #2332