BufferedFile: fclose at destruction

This is currently indirectly implied by src/bench/load_external.cpp:LoadExternalBlockFile
	"The file will be closed by LoadExternalBlockFile()."

src/streams.h

@@ -520,6 +520,10 @@ public:
             throw std::ios_base::failure("Rewind limit must be less than buffer size");
     }
 
+    ~BufferedFile() { fclose(); }
+
+    int fclose() { return m_src.fclose(); }
+
     //! check whether we're at the end of the source file
     bool eof() const {
         return m_read_pos == nSrcPos && m_src.feof();

src/test/fuzz/buffered_file.cpp

@@ -65,4 +65,8 @@ FUZZ_TARGET(buffered_file)
         }
         opt_buffered_file->GetPos();
     }
+    if (opt_buffered_file) {
+        opt_buffered_file->fclose();
+        opt_buffered_file.reset();
+    }
 }