Fix IO::Encoder#write when operating on long strings

spec/std/io/io_spec.cr

@@ -796,6 +796,19 @@ describe IO do
           io.encoding.should eq("UTF-8")
         end
 
+        it "handles long lines correctly with invalid: :skip" do
+          # Using both ASCII characters and a 26-byte Unicode characters to
+          # ensure we hit as many byte boundaries inside the Unicode characters
+          # as we can to get sufficient confidence in this test.
+          text = "test string 👩🏾‍🤝‍👨🏻" * 10240
+          io = IO::Memory.new
+          io.set_encoding "UTF-8", invalid: :skip
+          io << text
+
+          io.bytesize.should eq text.bytesize
+          io.to_slice.should eq text.to_slice
+        end
+
         it "does skips when converting to UTF-8" do
           io = SimpleIOMemory.new(Base64.decode_string("ey8qx+Tl8fwg7+Dw4Ozl8vD7IOLo5+jy4CovfQ=="))
           io.set_encoding("UTF-8", invalid: :skip)

src/crystal/iconv.cr

@@ -58,10 +58,19 @@ struct Crystal::Iconv
   def convert(inbuf : UInt8**, inbytesleft : LibC::SizeT*, outbuf : UInt8**, outbytesleft : LibC::SizeT*)
     {% if flag?(:freebsd) || flag?(:dragonfly) %}
       if @skip_invalid
-        return LibC.__iconv(@iconv, inbuf, inbytesleft, outbuf, outbytesleft, LibC::ICONV_F_HIDE_INVALID, out invalids)
+        err = LibC.__iconv(@iconv, inbuf, inbytesleft, outbuf, outbytesleft, LibC::ICONV_F_HIDE_INVALID, out invalids)
+        if err == Crystal::Iconv::ERROR && Errno.value != Errno::E2BIG
+          return err
+        else
+          return
+        end
       end
     {% end %}
-    {{ USE_LIBICONV ? LibIconv : LibC }}.iconv(@iconv, inbuf, inbytesleft, outbuf, outbytesleft)
+
+    err = {{ USE_LIBICONV ? LibIconv : LibC }}.iconv(@iconv, inbuf, inbytesleft, outbuf, outbytesleft)
+    if err == Crystal::Iconv::ERROR && Errno.value != Errno::E2BIG
+      return err
+    end
   end
 
   def handle_invalid(inbuf, inbytesleft)