Add assert()s to show that jumps always land an an instruction that is between

Add assert()s to show that jumps always land an an instruction that is between
1 and Vdbe.nOp-1.  Had these assert()s been in place before, they would have
caused an assertion fault for the byte-code error reported by
[forum:/forumpost/2482b32700|forum post 2482b32700].

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