shell: free cmdline before continue

This patch frees the cmdline when it's not empty but it doesn't contain
any actual character.

If the cmdline is just whitespaces or \r and \n, the loop continues
without freeing the cmdline string, so it's a leak.

Fix https://gitlab.gnome.org/GNOME/libxml2/-/issues/1009

shell.c

@@ -1138,8 +1138,11 @@ xmllintShell(xmlDoc *doc, const char *filename, FILE * output)
             command[i++] = *cur++;
         }
         command[i] = 0;
-        if (i == 0)
+        if (i == 0) {
+            free(cmdline);
+            cmdline = NULL;
             continue;
+        }
 
         /*
          * Parse the argument