Fix `IO#readchar` to return broken UTF-8 rather than `EOF` error.

The behavior is different from CRuby, but we believe this is a right
behavior for mruby, which only supports either ASCII or UTF-8
exclusively; fix #4983, ref #4982

```
$ printf '\xe3\x81' | ruby -e 'p STDIN.readchar'
"\xE3\x81"
```

```
$ printf '\xe3\x81' | mruby -e 'p STDIN.readchar'
"\xE3"
```

1 files changed, 2 insertions, 3 deletions

diff --git a/mrbgems/mruby-io/mrblib/io.rb b/mrbgems/mruby-io/mrblib/io.rb
index e43b81004..e597db886 100644
--- a/mrbgems/mruby-io/mrblib/io.rb
+++ b/mrbgems/mruby-io/mrblib/io.rb
@@ -284,15 +284,14 @@ class IO
   def readchar
     _read_buf
     _readchar(@buf)
-#    c = @buf[0]
-#    @buf[0] = ""
-#    c
   end
 
   def getc
     begin
       readchar
     rescue EOFError
+      c = @buf[0]
+      @buf[0,1]="" if c
       nil
     end
   end