Prevent failing stringstream to crash the export process

Text exporters are using string streams to hold the file content first and then write them to the file in a single pass. If for whatever reason the stream has the fail bit set, tellp() will return pos_type(-1), which in turn makes the subsequent write crash - at least on Windows systems. One reason for the stream being in fail state is when its size exceeds 2^31 bytes, even on 64-bit systems (i.e., when very large scenes get exported).

The fix is checking the fail() before even opening the file.

code/XFileExporter.cpp

@@ -78,6 +78,10 @@ void ExportSceneXFile(const char* pFile,IOSystem* pIOSystem, const aiScene* pSce
     // invoke the exporter
     XFileExporter iDoTheExportThing( pScene, pIOSystem, path, file, &props);
 
+    if (iDoTheExportThing.mOutput.fail()) {
+        throw DeadlyExportError("output data creation failed. Most likely the file became too large: " + std::string(pFile));
+    }
+    
     // we're still here - export successfully completed. Write result to the given IOSYstem
     std::unique_ptr<IOStream> outfile (pIOSystem->Open(pFile,"wt"));
     if(outfile == NULL) {