Cast to uschar when storing a char in an int that will be used as an index (#88)

* Cast to uschar when storing a char in an int that will be used as an index.
Fixes a heap underflow when the input char has the high bit set and
FS is a regex.

* Add regress test for underflow when RS is a regex and input is 8-bit.

b.c

@@ -684,7 +684,7 @@ bool fnematch(fa *pfa, FILE *f, char **pbuf, int *pbufsize, int quantum)
 						FATAL("stream '%.30s...' too long", buf);
 				buf[k++] = (c = getc(f)) != EOF ? c : 0;
 			}
-			c = buf[j];
+			c = (uschar)buf[j];
 			/* assert(c < NCHARS); */
 
 			if ((ns = pfa->gototab[s][c]) != 0)

bugs-fixed/rs_underflow.awk

@@ -0,0 +1 @@
+BEGIN { RS="zx" } { print $1 }

bugs-fixed/rs_underflow.in

@@ -0,0 +1 @@
+<EFBFBD>

bugs-fixed/rs_underflow.ok

@@ -0,0 +1 @@
+<EFBFBD>

lex.c

@@ -148,7 +148,7 @@ static int gettok(char **pbuf, int *psz)	/* get next input token */
 		strtod(buf, &rem);	/* parse the number */
 		if (rem == buf) {	/* it wasn't a valid number at all */
 			buf[1] = 0;	/* return one character as token */
-			retc = buf[0];	/* character is its own type */
+			retc = (uschar)buf[0];	/* character is its own type */
 			unputstr(rem+1); /* put rest back for later */
 		} else {	/* some prefix was a number */
 			unputstr(rem);	/* put rest back for later */