[LibOS] Set file handle to NULL after each shebang search iteration

Otherwise, this could lead to a "double `put_handle()`" problem when
the shebang script depth exceeded the limit.

Signed-off-by: Kailun Qin <kailun.qin@intel.com>

libos/src/libos_rtld.c

@@ -799,6 +799,7 @@ int load_and_check_exec(const char* path, const char* const* argv, struct libos_
         free(*curr_argv);
         free(curr_argv);
         put_handle(file);
+        file = NULL;
 
         curr_argv = new_argv;
     }