exec: case: Free word before apply_errexit_errreturn

Valgrind has detected a leak since the last commit. Valgrind seems to
think `word` is lost if `apply_errexit_errreturn` calls
`exit_shell_with_status` and never returns, whereas `word` would be
freed if `apply_errexit_errreturn` returned.

To suppress the false positive, this commit modifies the code so that
`word` is freed before `exit_shell_with_status` is called.

exec.c

@@ -1423,9 +1423,11 @@ void exec_case(const command_T *c, bool finally_exit)
     return;
 
 fail:
+    free(word);
     laststatus = Exit_EXPERROR;
     apply_errexit_errreturn(NULL);
-    goto done;
+    if (finally_exit)
+        exit_shell();
 }
 
 /* Executes the function definition. */