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/io/encoding.cr

@@ -40,7 +40,7 @@ class IO
         outbuf_ptr = outbuf.to_unsafe
         outbytesleft = LibC::SizeT.new(outbuf.size)
         err = @iconv.convert(pointerof(inbuf_ptr), pointerof(inbytesleft), pointerof(outbuf_ptr), pointerof(outbytesleft))
-        if err == Crystal::Iconv::ERROR
+        if err == Crystal::Iconv::ERROR && Errno.value != Errno::E2BIG
           @iconv.handle_invalid(pointerof(inbuf_ptr), pointerof(inbytesleft))
         end
         io.write(outbuf.to_slice[0, outbuf.size - outbytesleft])