Fix expand crash on invalid multialias root

[lukaszsamson]

I noticed this code crashes

iex(1)> defmodule A do
...(1)> @asd Foo
...(1)> alias @asd.{Bar}
...(1)> end
** (FunctionClauseError) no function clause matching in :elixir_aliases.do_concat/2    
    
    The following arguments were given to :elixir_aliases.do_concat/2:
    
        # 1
        [
          {{:., [line: 3], [Module, :__get_attribute__]}, [line: 3], [A, :asd, 3, true]},
          :Bar
        ]
    
        # 2
        "Elixir"
    
    (elixir 1.18.4) src/elixir_aliases.erl:183: :elixir_aliases.do_concat/2
    (elixir 1.18.4) src/elixir_aliases.erl:171: :elixir_aliases.concat/1
    (elixir 1.18.4) src/elixir_expand.erl:575: anonymous fn/8 in :elixir_expand.expand_multi_alias_call/7
    (elixir 1.18.4) src/elixir_expand.erl:701: :elixir_expand.mapfold/5
    (elixir 1.18.4) src/elixir_expand.erl:625: :elixir_expand.expand_block/5
    (elixir 1.18.4) src/elixir_expand.erl:48: :elixir_expand.expand/3
    (elixir 1.18.4) src/elixir_expand.erl:13: :elixir_expand.expand/3
    iex:1: (file)

Compare to:

iex(1)> defmodule A do
...(1)> @asd Foo
...(1)> alias @asd.Bar
...(1)> end
error: invalid alias: "@asd.Bar". If you wanted to define an alias, an alias must expand to an atom at compile time but it did not, you may use Module.concat/2 to build it at runtime. If instead you wanted to invoke a function or access a field, wrap the function or field name in double quotes
└─ iex:3: A (module)

** (CompileError) iex: cannot compile module A (errors have been logged)
    (elixir 1.18.4) src/elixir_expand.erl:71: :elixir_expand.expand/3
    (elixir 1.18.4) src/elixir_expand.erl:625: :elixir_expand.expand_block/5
    (elixir 1.18.4) src/elixir_expand.erl:48: :elixir_expand.expand/3
    (elixir 1.18.4) src/elixir_expand.erl:13: :elixir_expand.expand/3
    (elixir 1.18.4) src/elixir_expand.erl:634: :elixir_expand.expand_block/5
    (elixir 1.18.4) src/elixir_expand.erl:48: :elixir_expand.expand/3
    iex:1: (file)

[josevalim]

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