Add an assert() to the byte-code engine that goes off if the OP_Halt opcode

Add an assert() to the byte-code engine that goes off if the OP_Halt opcode
is invoked with SQLITE_INTERNAL.  This causes the RIGHT JOIN error
"Opcode jumps to ... which is outside the subroutine ..." to fail immediately,
causing it to come more readily to tester's attention.  There is at least
one testcase in test/fuzzdata8.db that asserts due to this change.

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