make agnostic, add defensive programming to bgetrune.

Reference: /n/sources/patch/applied/rune4-bio
Date: Thu Apr 25 00:55:34 CES 2013
Signed-off-by: geoff@plan9.bell-labs.com

--- /sys/src/libbio/bgetrune.c	Thu Apr 25 00:55:17 2013
+++ /sys/src/libbio/bgetrune.c	Thu Apr 25 00:55:17 2013
@@ -7,7 +7,7 @@
 {
 	int c, i;
 	Rune rune;
-	char str[4];
+	char str[UTFmax];
 
 	c = Bgetc(bp);
 	if(c < Runeself) {		/* one char */
@@ -15,19 +15,25 @@
 		return c;
 	}
 	str[0] = c;
+	bp->runesize = 0;
 
 	for(i=1;;) {
 		c = Bgetc(bp);
 		if(c < 0)
 			return c;
+		if (i >= sizeof str)
+			return Runeerror;
 		str[i++] = c;
 
 		if(fullrune(str, i)) {
+			/* utf is long enough to be a rune, but could be bad. */
 			bp->runesize = chartorune(&rune, str);
-			while(i > bp->runesize) {
-				Bungetc(bp);
-				i--;
-			}
+			if (rune == Runeerror)
+				bp->runesize = 0;	/* push back nothing */
+			else
+				/* push back bytes unconsumed by chartorune */
+				for(; i > bp->runesize; i--)
+					Bungetc(bp);
 			return rune;
 		}
 	}
--- /sys/src/libbio/bputrune.c	Thu Apr 25 00:55:17 2013
+++ /sys/src/libbio/bputrune.c	Thu Apr 25 00:55:17 2013
@@ -6,7 +6,7 @@
 Bputrune(Biobufhdr *bp, long c)
 {
 	Rune rune;
-	char str[4];
+	char str[UTFmax];
 	int n;
 
 	rune = c;