dict: fix unquote panic when backslash is the last character

[zsj8844]

The unquote function did not check bounds after advancing past a
backslash, causing an index out of range panic when the backslash
was the final character in the string.

Add a bounds check and break out of the loop when the backslash
has no following character to escape.

Fixes golang/go#259

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Patch Set 1:

(1 comment)

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Message from 张世杰:

Patch Set 2:

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Patch Set 4:

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Message from Nicholas Husin:

Patch Set 4: Hold+1

(1 comment)

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