patch 8.2.2848: crash whn calling partial

Problem:    Crash whn calling partial.
Solution:   Check for NULL pointer. (Dominique Pellé, closes #8202)
diff --git a/src/eval.c b/src/eval.c
index aa3d0a1..0d4f5fe 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -4284,10 +4284,13 @@
     char_u *
 partial_name(partial_T *pt)
 {
-    if (pt->pt_name != NULL)
-	return pt->pt_name;
-    if (pt->pt_func != NULL)
-	return pt->pt_func->uf_name;
+    if (pt != NULL)
+    {
+	if (pt->pt_name != NULL)
+	    return pt->pt_name;
+	if (pt->pt_func != NULL)
+	    return pt->pt_func->uf_name;
+    }
     return (char_u *)"";
 }