gguf : fix resource leaks (#6061)

There several places where a gguf context is allocated. A call to gguf_free
is missing in some error paths. Also on linux, llama-bench was missing a
fclose.

examples/llama-bench/llama-bench.cpp

@@ -103,6 +103,7 @@ static std::string get_cpu_info() {
                 }
             }
         }
+        fclose(f);
     }
 #endif
     // TODO: other platforms