dnsmasq: Fix potential dnsmasq crash with TCP

dnsmasq: Fix potential dnsmasq crash with TCP

This is a backport from the dnsmasq master which should fix a bug which
could cause a crash in dnsmasq.

I saw the following crashes in my log:
[522413.117215] do_page_fault(): sending SIGSEGV to dnsmasq for invalid read access from 2a001450
[522413.124464] epc = 004197f1 in dnsmasq[400000+23000]
[522413.129459] ra  = 004197ef in dnsmasq[400000+23000]
This is happening in blockdata_write() when block->next is
dereferenced, but I am not sure if this is related to this problem or if
this is a different problem. I am unable to reproduce this problem.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>