fix off-by one error in a2p

The str_gets() function, when encountering a newline character, checked to
see if the previous char was a \ escape. For a blank line, the check would
read the char at the position one before the start of the buffer. There
was a test to avoid this, but it was off-by-one.

AUTHORS

@@ -930,6 +930,7 @@ Petter Reinholdtsen		<pere@hungry.com>
 Phil Lobbes			<phil@perkpartners.com>
 Phil Monsen			<philip.monsen@pobox.com>
 Philip Boulain			<philip.boulain@smoothwall.net>
+Philip Guenther                 <guenther@openbsd.org>
 Philip Hazel			<ph10@cus.cam.ac.uk>
 Philip M. Gollucci		<pgollucci@p6m7g8.com>
 Philip Newton			<pne@cpan.org>

x2p/str.c

@@ -200,7 +200,7 @@ str_gets(STR *str, FILE *fp)
     for (;;) {
 	while (--cnt >= 0) {
 	    if ((*bp++ = *ptr++) == newline) {
-		if (bp <= str->str_ptr || bp[-2] != '\\')
+		if (bp <= str->str_ptr + 1 || bp[-2] != '\\')
 		    goto thats_all_folks;
 		else {
 		    line++;