Fix 'send-string-to-terminal' writing very long strings

* src/dispnew.c (Fsend_string_to_terminal): Prevent partial writes
by blocking SIGIO while 'fwrite' runs.  (Bug#44320)

src/dispnew.c

@@ -5904,8 +5904,12 @@ when TERMINAL is nil.  */)
 	}
       out = tty->output;
     }
+  /* STRING might be very long, in which case fwrite could be
+     interrupted by SIGIO.  So we temporarily block SIGIO.  */
+  unrequest_sigio ();
   fwrite (SDATA (string), 1, SBYTES (string), out);
   fflush (out);
+  request_sigio ();
   unblock_input ();
   return Qnil;
 }