mmime: print_header: only force qp-encoding for overlong lines

Lines SHOULD not be longer than 78 chars, and we try to fold like that,
but we only enforce qp-encoding for 7-bit safe lines if they would be
longer than 998, which they MUST not be.

mmime.c

@@ -250,7 +250,7 @@ print_header(char *line) {
 		}
 
 		if (!highbit) {
-			if (e-s >= 78)
+			if (e-s >= 998)
 				goto force_qp;
 			if (e-s >= 78 - linelen) {
 				// wrap in advance before long word