Fix crash with unmodified bit builder

Author: lpil

src/gleam/bit_builder.gleam

@@ -74,7 +74,7 @@ pub external fn concat(List(BitBuilder)) -> BitBuilder =
 /// Runs in constant time.
 ///
 pub external fn from_bit_string(BitString) -> BitBuilder =
-  "gleam_stdlib" "identity"
+  "gleam_stdlib" "wrap_list"
 
 /// Turns an builder into a bit string.
 ///

src/gleam_stdlib.erl

@@ -11,7 +11,7 @@
          string_pad/4, decode_tuple2/1, decode_map/1, bit_string_int_to_u32/1,
          bit_string_int_from_u32/1, bit_string_append/2, bit_string_part_/3,
          decode_bit_string/1, compile_regex/2, regex_match/2, regex_split/2,
-         regex_scan/2, base_decoded4/1]).
+         regex_scan/2, base_decoded4/1, wrap_list/1]).
 
 should_equal(Actual, Expected) -> ?assertEqual(Expected, Actual).
 should_not_equal(Actual, Expected) -> ?assertNotEqual(Expected, Actual).
@@ -205,3 +205,6 @@ base_decoded4(S) ->
   try {ok, base64:decode(S)}
   catch error:badarith -> {error, nil}
   end.
+
+wrap_list(X) when is_list(X) -> X;
+wrap_list(X) -> [X].

test/gleam/bit_string_builder_test.gleam renamed to test/gleam/bit_builder_test.gleam

@@ -56,3 +56,10 @@ pub fn concat_test() {
   |> bit_builder.to_bit_string
   |> should.equal(<<1, 2, 3, 4, 5, 6>>)
 }
+
+pub fn from_string_test() {
+  // Regression test: no additional modification of the builder
+  bit_builder.from_bit_string(<<>>)
+  |> bit_builder.to_bit_string
+  |> should.equal(<<>>)
+}