Fix compiler crash on invalid nested bitstring

[lukaszsamson]

This PR fixes a crash when handling invalid bitstring:

iex(1)> Code.compile_string("""
        defmodule Repro do
          def run do
            <<(<<1>>)::integer>>
          end
        end
        """)
error: conflicting type specification for bit field: "integer" and "bitstring"
└─ nofile:3:14: Repro.run/0

** (CaseClauseError) no case clause matching:

    {:integer, [], nil}

    (elixir 1.20.0-rc.4) src/elixir_bitstring.erl:105: :elixir_bitstring.concat_or_prepend_bitstring/6
    (elixir 1.20.0-rc.4) src/elixir_bitstring.erl:50: :elixir_bitstring.expand/8
    (elixir 1.20.0-rc.4) src/elixir_bitstring.erl:25: :elixir_bitstring.expand/5
    (elixir 1.20.0-rc.4) src/elixir_clauses.erl:195: :elixir_clauses.def/3
    (elixir 1.20.0-rc.4) src/elixir_def.erl:198: :elixir_def."-store_definition/10-lc$^0/1-0-"/3
    (elixir 1.20.0-rc.4) src/elixir_def.erl:199: :elixir_def.store_definition/10
    nofile:2: (module)
    iex:1: (file)

With the changes:

iex(1)> Code.compile_string("""
        defmodule Repro do
          def run do
            <<(<<1>>)::integer>>
          end
        end
        """)
error: conflicting type specification for bit field: "integer" and "bitstring"
└─ nofile:3:14: Repro.run/0

** (CompileError) nofile: cannot compile module Repro (errors have been logged)
    (elixir 1.20.0-rc.4) src/elixir_module.erl:192: anonymous fn/11 in :elixir_module.compile/7
    iex:1: (file)

[josevalim]

💚 💙 💜 💛 ❤️