js: fix topic and payload offsets for publish frames

[jrobsonchase]

There were a few things wrong here:

• The search for the topic's null terminator was starting from the beginning of the payload, rather than after the sender's null terminator
• The topic slice was also starting from the beginning of the payload
• The i cursor was getting an extra +1, presumably to skip the null bytes for both the sender and the topic, even though the sender's byte was already being skipped a couple of lines down

Now, the sender null byte is skipped earlier for clarity and the search/slice for the topic starts in the right place.

[divi255]

thank you. merged. will be released next week after tests

[divi255]

published in js client 0.1.2