mailinfo.c: move side-effects outside of assert

Since 6b4b013 (mailinfo: handle in-body header continuations,
2016-09-20, v2.11.0) mailinfo.c has contained new code with an
assert of the form:

	assert(call_a_function(...))

The function in question, check_header, has side effects.  This
means that when NDEBUG is defined during a release build the
function call is omitted entirely, the side effects do not
take place and tests (fortunately) start failing.

Since the only time that mi->inbody_header_accum is appended to is
in check_inbody_header, and appending onto a blank
mi->inbody_header_accum always happens when is_inbody_header is
true, this guarantees a prefix that causes check_header to always
return true.

Therefore replace the assert with an if !check_header + DIE
combination to reflect this.

Helped-by: Jonathan Tan <[email protected]>
Helped-by: Jeff King <[email protected]>
Acked-by: Johannes Schindelin <[email protected]>
Signed-off-by: Kyle J. McKay <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: mackyle

mailinfo.c

@@ -628,7 +628,8 @@ static void flush_inbody_header_accum(struct mailinfo *mi)
 {
 	if (!mi->inbody_header_accum.len)
 		return;
-	assert(check_header(mi, &mi->inbody_header_accum, mi->s_hdr_data, 0));
+	if (!check_header(mi, &mi->inbody_header_accum, mi->s_hdr_data, 0))
+		die("BUG: inbody_header_accum, if not empty, must always contain a valid in-body header");
 	strbuf_reset(&mi->inbody_header_accum);
 }