Fix rb_str_conv_enc_opts when --enable-frozen-string-literal Ruby option is specified

Author: andrykonchin

lib/truffle/truffle/cext.rb

@@ -881,7 +881,7 @@ def rb_str_conv_enc_opts(str, from, to, ecflags, ecopts)
     end
 
     ec = Encoding::Converter.new(from, to, ecopts || ecflags)
-    dest = ''
+    dest = +''
     # This C API will (unlike primitive convert) not alter the source
     # string, so we need to duplicate it.
     status = ec.primitive_convert str.dup, dest, nil, nil