Fixes GH-10715
When a string starting with a NUL character is passed to
phpdbg_vprint(), the vasprintf() will return that 0 characters have been
printed. This causes msglen == 0. When phpdbg_process_print() is called
with a message of length 0, the -1 to check for '\n' will perform an out
of bounds read. Since nothing is printed anyway for msglen == 0, it
seems best to just skip the printing routine for this case.
Closes GH-10720.
0f21cbc57c