back-port from 5.1.

Bug#35480: BOM detection code crashes mysql CLI with zero-sized input
      
MySQL client crashed if no input was passed to it.

client/mysql.cc

@@ -1779,7 +1779,7 @@ static int read_and_execute(bool interactive)
         the very beginning of a text file when
         you save the file using "Unicode UTF-8" format.
       */
-      if (!line_number &&
+      if (line && !line_number &&
            (uchar) line[0] == 0xEF &&
            (uchar) line[1] == 0xBB &&
            (uchar) line[2] == 0xBF)