lib-mail: message-parser - Fix out-of-bounds read on empty preamble

[nishat-06]

Re-parsing a multipart message from cached parts with
MESSAGE_PARSER_FLAG_INCLUDE_MULTIPART_BLOCKS, on a message whose MIME
part has an empty preamble (the boundary is the first thing in the body):

block: hdr=(nil) size=18446744073709551614
Panic: nearest_power(18446744073709551615): calculation would overflow

preparsed_parse_prologue_more() scans backwards from the boundary's
trailing CRLF for the newline that precedes --boundary. With no preamble
the boundary sits at the very start of the prologue block, so the scan runs
off the front and exits with cur == block_r->data - 1. data[-1] happens
to be the \n ending the multipart part header, so the -- check passes on
data[0]/data[1] and block_r->size = cur - block_r->data underflows to a
near-SIZE_MAX value. The prologue block is then handed to the consumer with
that size, which over-reads (the panic above is a consumer appending it).

Reachable via message_parser_init_from_parts() when a caller re-parses a
message against its cached message_part tree and asks for multipart blocks.

Handle the no-preamble case explicitly: when the backward scan finds no
newline, confirm the block starts with -- and return an empty prologue.
Regression test added to test-message-parser.c.

[cmouse]

Thank you, we'll take a look.