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

src/bin/psql/command.c

@@ -480,7 +480,7 @@ exec_command_bind(PsqlScanState scan_state, bool active_branch)
 				nalloc = nalloc ? nalloc * 2 : 1;
 				pset.bind_params = pg_realloc_array(pset.bind_params, char *, nalloc);
 			}
-			pset.bind_params[nparams - 1] = pg_strdup(opt);
+			pset.bind_params[nparams - 1] = opt;
 		}
 
 		pset.bind_nparams = nparams;