Zed_char: improve first_core

Author: kandu

src/zed_char.ml

@@ -101,10 +101,10 @@ let first_core ?(trim=false) uChars=
   match uChars with
   | []-> None, []
   | uChar::_->
-    if not trim && is_combining_mark uChar then
-      None, uChars
-    else
+    if trim || is_printable_core uChar then
       aux uChars
+    else
+      None, uChars
 
 let rec subsequent uChars=
   match uChars with