WebSockets: fix invalid read when sending large messages

We use GByteArray, which can be reallocated, so be careful when
keeping track of the current position in a message not to use
potentially dangling pointers.

Fixes #160
1 job for master in 1 minute and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed fedora-meson-x86_64 #402192
ipv6

00:01:02