Fix Linux Release build compiler warning

src/qgcunittest/MockMavlinkFileServer.cc

@@ -139,7 +139,7 @@ void MockMavlinkFileServer::_openCommand(QGCUASFileManager::Request* request, ui
     
     // Data contains file length
     response.hdr.size = sizeof(uint32_t);
-    *((uint32_t*)response.data) = _readFileLength;
+    response.openFileLength = _readFileLength;
     
     _emitResponse(&response, outgoingSeqNumber);
 }

src/uas/QGCUASFileManager.cc

@@ -107,8 +107,7 @@ void QGCUASFileManager::_openAckResponse(Request* openAck)
     
     // File length comes back in data
     Q_ASSERT(openAck->hdr.size == sizeof(uint32_t));
-    uint32_t fileLength = *((uint32_t*)&openAck->data[0]);
-    emit openFileLength(fileLength);
+    emit openFileLength(openAck->openFileLength);
 
     _readOffset = 0;                // Start reading at beginning of file
     _readFileAccumulator.clear();   // Start with an empty file

src/uas/QGCUASFileManager.h

@@ -71,9 +71,16 @@ protected:
     struct Request
     {
         struct RequestHeader hdr;
-        // The entire Request must fit into the data member of the mavlink_encapsulated_data_t structure. We use as many leftover bytes
-        // after we use up space for the RequestHeader for the data portion of the Request.
-        uint8_t data[sizeof(((mavlink_encapsulated_data_t*)0)->data) - sizeof(RequestHeader)];
+
+        // We use a union here instead of just casting (uint32_t)&data[0] to not break strict aliasing rules
+        union {
+            // The entire Request must fit into the data member of the mavlink_encapsulated_data_t structure. We use as many leftover bytes
+            // after we use up space for the RequestHeader for the data portion of the Request.
+            uint8_t data[sizeof(((mavlink_encapsulated_data_t*)0)->data) - sizeof(RequestHeader)];
+
+            // File length returned by Open command
+            uint32_t openFileLength;
+        };
     };
 
     enum Opcode