Assert that the file is really closed after closing it in importer.

Should harden plugin implementations a bit more.

src/Audio/AbstractImporter.cpp

@@ -76,7 +76,10 @@ void AbstractImporter::doOpenFile(const std::string& filename) {
 }
 
 void AbstractImporter::close() {
-    if(isOpened()) doClose();
+    if(isOpened()) {
+        doClose();
+        CORRADE_INTERNAL_ASSERT(!isOpened());
+    }
 }
 
 Buffer::Format AbstractImporter::format() const {

src/Text/AbstractFont.cpp

@@ -100,7 +100,10 @@ void AbstractFont::doOpenFile(const std::string& filename, const Float size) {
 }
 
 void AbstractFont::close() {
-    if(isOpened()) doClose();
+    if(isOpened()) {
+        doClose();
+        CORRADE_INTERNAL_ASSERT(!isOpened());
+    }
 }
 
 UnsignedInt AbstractFont::glyphId(const char32_t character) {

src/Trade/AbstractImporter.cpp

@@ -76,7 +76,10 @@ void AbstractImporter::doOpenFile(const std::string& filename) {
 }
 
 void AbstractImporter::close() {
-    if(isOpened()) doClose();
+    if(isOpened()) {
+        doClose();
+        CORRADE_INTERNAL_ASSERT(!isOpened());
+    }
 }
 
 Int AbstractImporter::defaultScene() {