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awk: fix out of bounds write with large fields (thanks kristo)

; dd < /dev/zero -bs 4 -count 8k | tr '\x0' A | awk '{a=$1}'

Solution copied from onetrueawk.

--- a/sys/src/cmd/awk/lib.c

+++ b/sys/src/cmd/awk/lib.c

@@ -261,7 +261,7 @@

 	n = strlen(r);

 	if (n > fieldssize) {

 		xfree(fields);

-		if ((fields = (char *) malloc(n+1)) == nil)

+		if ((fields = (char *) malloc(n+2)) == nil)  /* possibly 2 final \0s */

 			FATAL("out of space for fields in fldbld %d", n);

 		fieldssize = n;

 	}