patch 8.0.0399: crash when using balloon_show() when not supported

Problem:    Crash when using balloon_show() when not supported. (Hirohito
            Higashi)
Solution:   Check for balloonEval not to be NULL. (Ken Takata)

src/evalfunc.c

@@ -1375,6 +1375,7 @@ f_atan2(typval_T *argvars, typval_T *rettv)
     static void
 f_balloon_show(typval_T *argvars, typval_T *rettv UNUSED)
 {
+    if (balloonEval != NULL)
 	gui_mch_post_balloon(balloonEval, get_tv_string_chk(&argvars[0]));
 }
 #endif

src/testdir/test_functions.vim

@@ -466,3 +466,8 @@ func Test_getbufvar()
 
   set fileformats&
 endfunc
+
+func Test_balloon_show()
+  " This won't do anything but must not crash either.
+  call balloon_show('hi!')
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    399,
 /**/
     398,
 /**/