notmuch: Fix off-by-one errors if a header is >200 characters long.

notmuch: Fix off-by-one errors if a header is >200 characters long.

If a single header is more than 200 characters long a set of 'off by
one' errors cause memory corruption.

When allocating memory with:
     a = malloc (len);
the last usable byte of the memory is 'a + len - 1' rather than 'a +
len'.

Fix the same bug when calculating the current offset should the buffer
used for collecting the output header need to be reallocated.