patch 8.2.3396: when libcall() fails invalid pointer may be used

Problem:    When libcall() fails invalid pointer may be used.
Solution:   Initialize the string to NULL. (Yasuhiro Matsumoto, closes #8829)

src/evalfunc.c

@@ -6689,9 +6689,14 @@ libcall_common(typval_T *argvars UNUSED, typval_T *rettv, int type)
 	if (argvars[2].v_type == VAR_STRING)
 	    string_in = argvars[2].vval.v_string;
 	if (type == VAR_NUMBER)
+	{
 	    string_result = NULL;
+	}
 	else
+	{
+	    rettv->vval.v_string = NULL;
 	    string_result = &rettv->vval.v_string;
+	}
 	if (mch_libcall(argvars[0].vval.v_string,
 			     argvars[1].vval.v_string,
 			     string_in,

src/version.c

@@ -755,6 +755,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3396,
 /**/
     3395,
 /**/