mailinfo.c: move side-effects outside of assert

Since 6b4b013f18 (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 <jonathantanmy@google.com> Helped-by: Jeff King <peff@peff.net> Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Kyle J. McKay <mackyle@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>