(scan_message): Don't set first elements of fromheader and subjheader to
'\0'; that was pointless. Don't assign to line[--len], only decrement
len; line is a private buffer beloging to fgetln and we should not be
scribbling in it; this was also pointless anyway. Set
fromheader[fromcount] to '\0' after copying the characters from line
into it; do the same for subjheader.
(frm): Make fromheader and subjheader automatic variables, no longer
use malloc. Also, initialize the whole buffers (fixing memory issues
picked up by valgrind).