rc: fix rc API accepting an out of range number and overflowing 64 bits

float64(math.MaxInt64) rounds up to 2^63, so x > math.MaxInt64 in
GetInt64 lets 2^63 through to int64(x), which is out of range.

(cherry picked from commit 976d05e1dd)
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f