list_features(): Hack around infinite loop.

msg_putchar() is not updating msg_col, this causes an infinite loop. Observed
with execute('version') *nested* in another execute(), in particular this line:
    let buildtype = matchstr(execute('version'), '\v\cbuild type:?\s*[^\n\r\t ]+')
when called by :CheckHealth (see runtime/autoload/health/nvim ..
s:check_performance()).

But invoking some variation of execute('...execute("version")') is not enough to
provoke the bug, maybe it needs to be in a user function?

src/nvim/version.c

@@ -2570,7 +2570,11 @@ static void list_features(void)
         }
       } else {
         while (msg_col % width) {
+          int old_msg_col = msg_col;
           msg_putchar(' ');
+          if (old_msg_col == msg_col) {
+            break;  // XXX: Avoid infinite loop.
+          }
         }
       }
     } else {