shared/shell: Fix memory leak by generator for submenu

Since asprintf() allocates new memory when a submenu command is
complemented, the memory leak occurs as below:

   8 bytes in 1 blocks are definitely lost in loss record 18 of 179
      at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
      by 0x5679C99: strdup (strdup.c:42)
      by 0x13B1E7: find_cmd.constprop.2 (shell.c:597)
      by 0x13B2D1: cmd_generator (shell.c:646)
      by 0x53B976D: rl_completion_matches (in /lib/x86_64-linux-gnu/libreadline.so.7.0)
      by 0x13BA91: shell_completion (shell.c:777)
      by 0x53B98B6: ??? (in /lib/x86_64-linux-gnu/libreadline.so.7.0)
      by 0x53B9A99: rl_complete_internal (in /lib/x86_64-linux-gnu/libreadline.so.7.0)
      by 0x53B02EE: _rl_dispatch_subseq (in /lib/x86_64-linux-gnu/libreadline.so.7.0)
      by 0x53B07B5: readline_internal_char (in /lib/x86_64-linux-gnu/libreadline.so.7.0)
      by 0x53C8F84: rl_callback_read_char (in /lib/x86_64-linux-gnu/libreadline.so.7.0)
      by 0x13AD80: input_read (shell.c:1065)
1 file changed