closes https://github.com/assimp/assimp/issues/901

code/DefaultIOStream.cpp

@@ -55,6 +55,7 @@ DefaultIOStream::~DefaultIOStream()
 {
     if (mFile) {
         ::fclose(mFile);
+        mFile = nullptr;
     }
 }
 
@@ -111,7 +112,7 @@ size_t DefaultIOStream::FileSize() const
 
     if (SIZE_MAX == cachedSize) {
 
-        // Although fseek/ftell would allow us to reuse the exising file handle here,
+        // Although fseek/ftell would allow us to reuse the existing file handle here,
         // it is generally unsafe because:
         //  - For binary streams, it is not technically well-defined
         //  - For text files the results are meaningless
@@ -125,12 +126,14 @@ size_t DefaultIOStream::FileSize() const
         if (0 != err)
             return 0;
         cachedSize = (size_t) (fileStat.st_size);
-#else
+#elif defined __gnu_linux__
         struct stat fileStat;
         int err = stat(mFilename.c_str(), &fileStat );
         if (0 != err)
             return 0;
         cachedSize = (size_t) (fileStat.st_size);
+#else
+#   error "Unknown platform"
 #endif
     }
     return cachedSize;