patch 9.0.1166: code is indented more than necessary

Problem:    Code is indented more than necessary.
Solution:   Use an early return where it makes sense. (Yegappan Lakshmanan,
            closes #11792)
diff --git a/src/diff.c b/src/diff.c
index 41cfefe..ec8c8f1 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -559,14 +559,14 @@
     diff_T	*dnew;
 
     dnew = ALLOC_ONE(diff_T);
-    if (dnew != NULL)
-    {
-	dnew->df_next = dp;
-	if (dprev == NULL)
-	    tp->tp_first_diff = dnew;
-	else
-	    dprev->df_next = dnew;
-    }
+    if (dnew == NULL)
+	return NULL;
+
+    dnew->df_next = dp;
+    if (dprev == NULL)
+	tp->tp_first_diff = dnew;
+    else
+	dprev->df_next = dnew;
     return dnew;
 }