When parsing OP_PUSHDATAX commands, the the length of data might not require
the size integer of OP_PUSHDATAX. For instance, you might write 1 byte, and yet
use OP_PUSHDATA4. We need to record which OP_PUSHDATAX was used so that when we
write the buffer back out, we can write the same one. Also, the claimed length
may be different. For instance, we may OP_PUSHDATA of length 100 to the stack,
but there may only be 50 bytes left in the script. In that case, buf.length and
chunk.len will be different. I'm not sure if that would be considered a valid
script, but in any case, for script analysis, we need both values.
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