Fix !defined(SK_XML) builds

Add guards for dependent code.

TBR=
Change-Id: I49c0c90a2defc6f39589721f95562e9e0f17db13
Reviewed-on: https://skia-review.googlesource.com/144610
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/tools/skpbench/skpbench.cpp b/tools/skpbench/skpbench.cpp
index 55fbc00..8084ca9 100644
--- a/tools/skpbench/skpbench.cpp
+++ b/tools/skpbench/skpbench.cpp
@@ -533,6 +533,7 @@
     return recorder.finishRecordingAsPicture();
 #endif
     exitf(ExitErr::kData, "SK_XML is disabled; cannot open svg file %s", filename);
+    return nullptr;
 }
 
 bool mkdir_p(const SkString& dirname) {