fix: the paredit-splice should not backward 1 form if current point already at open-char

[sakurawald]

This pr fixes the matched level of open-char and closed-char.
To fix cases like: #1612

Before this pr: the paredit-splice treat string between open-char and closed-char and closed-char as the same form, but treat the open-char as the (form-offset (current-point) -1).

After this pr: the paredit-splice treat open-char, string between open-char and closed-char and closed-char as the same form.

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For this test-case:

Before this pr:

After this pr:

[sakurawald]

Closed as this is treated as a feature.