fix skiaserve crash

tools/skiaserve/skiaserve.cpp

Index: tools/skiaserve/skiaserve.cpp
diff --git a/tools/skiaserve/skiaserve.cpp b/tools/skiaserve/skiaserve.cpp
index 6b93429764a56328873f6d8c0fb80776d918cf62..409809b91bc43e32c7515611c2e792fb0014f876 100644
--- a/tools/skiaserve/skiaserve.cpp
+++ b/tools/skiaserve/skiaserve.cpp
@@ -210,6 +210,7 @@ public:
             SkData* data = request->fPNG.get();
             return SendData(connection, data, "image/png");
         }
+
         return MHD_NO;
     }
 };
@@ -257,6 +258,10 @@ public:
             return MHD_YES;
         }
 
+        // clear upload context
+        delete request->fUploadContext;
+        request->fUploadContext = nullptr;
+
         return SendTemplate(connection, true, "/");
     }
 };