patch 8.1.1219: not checking for NULL return from alloc()

Problem:    Not checking for NULL return from alloc().
Solution:   Add checks. (Martin Kunev, closes #4303, closes #4174)
diff --git a/src/ops.c b/src/ops.c
index e939f04..2aa18c4 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -6170,21 +6170,25 @@
     y_ptr->y_size = linecount;
     y_ptr->y_time_set = timestamp;
     if (linecount == 0)
-	y_ptr->y_array = NULL;
-    else
     {
-	y_ptr->y_array =
-		   (char_u **)alloc((unsigned)(linecount * sizeof(char_u *)));
-	for (i = 0; i < linecount; i++)
+	y_ptr->y_array = NULL;
+	return;
+    }
+    y_ptr->y_array = (char_u **)alloc((unsigned)(linecount * sizeof(char_u *)));
+    if (y_ptr->y_array == NULL)
+    {
+	y_ptr->y_size = 0; // ensure object state is consistent
+	return;
+    }
+    for (i = 0; i < linecount; i++)
+    {
+	if (vp[i + 6].bv_allocated)
 	{
-	    if (vp[i + 6].bv_allocated)
-	    {
-		y_ptr->y_array[i] = vp[i + 6].bv_string;
-		vp[i + 6].bv_string = NULL;
-	    }
-	    else
-		y_ptr->y_array[i] = vim_strsave(vp[i + 6].bv_string);
+	    y_ptr->y_array[i] = vp[i + 6].bv_string;
+	    vp[i + 6].bv_string = NULL;
 	}
+	else
+	    y_ptr->y_array[i] = vim_strsave(vp[i + 6].bv_string);
     }
 }