Fix small memory leak in psql's \bind command

Fix small memory leak in psql's \bind command

psql_scan_slash_option() returns a malloc()'d result through a
PQExpBuffer, and exec_command_bind() was doing an extra allocation of
this option for no effect.

Introduced in 5b66de3.

Author: Kyotaro Horiguchi
Reviewed-by: Corey Huinker
Discussion: https://postgr.es/m/20230221.115555.89096938631423206.horikyota.ntt@gmail.com