Fix minor memory leak in psftp batch mode.

Spotted by Leak Sanitiser, while I was investigating the PSFTP /
proftpd issue mentioned in the previous commit (with ASan on as
usual).

The two very similar loops that read PSFTP commands from the
interactive prompt and a batch file differed in one respect: only one
of them remembered to free the command afterwards. Now I've moved the
freeing code out into a subroutine that both loops can use.

psftp.c

@@ -2307,6 +2307,16 @@ struct sftp_command *sftp_getcmd(FILE *fp, int mode, int modeflags)
     return cmd;
 }
 
+static void sftp_cmd_free(struct sftp_command *cmd)
+{
+    if (cmd->words) {
+        for (size_t i = 0; i < cmd->nwords; i++)
+            sfree(cmd->words[i]);
+        sfree(cmd->words);
+    }
+    sfree(cmd);
+}
+
 static int do_sftp_init(void)
 {
     struct sftp_packet *pktin;
@@ -2381,13 +2391,7 @@ int do_sftp(int mode, int modeflags, char *batchfile)
             if (!cmd)
                 break;
             ret = cmd->obey(cmd);
-            if (cmd->words) {
-                int i;
-                for(i = 0; i < cmd->nwords; i++)
-                    sfree(cmd->words[i]);
-                sfree(cmd->words);
-            }
-            sfree(cmd);
+            sftp_cmd_free(cmd);
             if (ret < 0)
                 break;
         }
@@ -2404,6 +2408,7 @@ int do_sftp(int mode, int modeflags, char *batchfile)
             if (!cmd)
                 break;
             ret = cmd->obey(cmd);
+            sftp_cmd_free(cmd);
             if (ret < 0)
                 break;
             if (ret == 0) {