Fix for issue #9775. Added check for open() returning a NULL pointer.

src/util.cpp

@@ -214,13 +214,14 @@ void OpenDebugLog()
     assert(vMsgsBeforeOpenLog);
     boost::filesystem::path pathDebug = GetDataDir() / "debug.log";
     fileout = fopen(pathDebug.string().c_str(), "a");
-    if (fileout) setbuf(fileout, NULL); // unbuffered
-
+    if (fileout) {
+        setbuf(fileout, NULL); // unbuffered
         // dump buffered messages from before we opened the log
         while (!vMsgsBeforeOpenLog->empty()) {
             FileWriteStr(vMsgsBeforeOpenLog->front(), fileout);
             vMsgsBeforeOpenLog->pop_front();
         }
+    }
 
     delete vMsgsBeforeOpenLog;
     vMsgsBeforeOpenLog = NULL;