Reject sub-minimum ErrorResponse length in read_kind_from_backend.

Emond Papegaaij reported a bug in read_kind_from_backend.

read_kind_from_backend() handles a backend ErrorResponse on the V3
path by reading the 4-byte length field, palloc'ing a buffer sized at
ntohl(len), and immediately memcpy()ing all 4 bytes of the
network-order length back into that buffer. When a backend (malicious,
buggy, or speaking a non-V3 dialect that has slipped past earlier
checks) sends a length below 4 ― i.e. in {0,1,2,3} ― the palloc
chunk is smaller than sizeof(len) and the memcpy overruns the heap
allocation. The follow-up `len -= 4` then underflows to a huge
unsigned value, which the subsequent `repalloc(unread_p, sizeof(len) +
len)` may wrap, and pool_read2() is asked for a colossal payload ―
all on top of an already-corrupted heap.

Reject any ntohl(len) below sizeof(len) up front with ereport(ERROR,
...). The error path is the existing longjmp-based one used by every
other malformed-input check in this function, so the connection is
torn down cleanly without touching unread_p.

Patch attached.

Regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp