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Fix memory leak in string assignment to built-ins

The new string value is always freeable (strdup'd by tostring), so
never mark it as DONTFREE.

$ paste <(./leak.sh master.out) <(./leak.sh fixed.out)
 2564	 2464
 7260	 2476
11948	 2476
16636	 2476
21324	 2476
26012	 2476
30700	 2476
35388	 2476
40076	 2476
44760	 2476
49448	 2476

$ cat leak.sh
#/bin/sh

awk=${1:-nawk}

# Print the initial resident set size
# Assign each record to FS
# Print rsz every 100,000 records
# Exit after a million

yes abcdefghijklmnopqrstuvwxyz | $awk -v mem='ps -p $PPID -o rsz=' '
    BEGIN            { system(mem) }
                     { FS = $0 }
    NR % 100000 == 0 { system(mem) }
    NR == 1000000    { exit }
'

--- a/FIXES

+++ b/FIXES

@@ -25,6 +25,10 @@

 This file lists all bug fixes, changes, etc., made since the AWK book

 was sent to the printers in August, 1987.

+May 23, 2022:

+	Memory leak when assigning a string to some of the built-in

+	variables. Thanks to Miguel Pineiro Jr. <mpj@pineiro.cc>.

+

 Mar 3, 2022:

 	Fixed file management memory leak that appears to have been

 	there since the files array was first initialized with stdin,

--- a/main.c

+++ b/main.c

@@ -22,7 +22,7 @@

 THIS SOFTWARE.

 ****************************************************************/

-const char	*version = "version 20220303";

+const char	*version = "version 20220523";

 #define DEBUG

 #include <stdio.h>

--- a/tran.c

+++ b/tran.c

@@ -70,18 +70,6 @@

 extern Cell **fldtab;

-static void

-setfree(Cell *vp)

-{

-	if (&vp->sval == FS || &vp->sval == RS ||

-	    &vp->sval == OFS || &vp->sval == ORS ||

-	    &vp->sval == OFMT || &vp->sval == CONVFMT ||

-	    &vp->sval == FILENAME || &vp->sval == SUBSEP)

-		vp->tval |= DONTFREE;

-	else

-		vp->tval &= ~DONTFREE;

-}

-

 void syminit(void)	/* initialize symbol table with builtin vars */

 {

 	literal0 = setsymtab("0", "0", 0.0, NUM|STR|CON|DONTFREE, symtab);

@@ -377,10 +365,9 @@

 	t = s ? tostring(s) : tostring("");	/* in case it's self-assign */

 	if (freeable(vp))

 		xfree(vp->sval);

-	vp->tval &= ~(NUM|CONVC|CONVO);

+	vp->tval &= ~(NUM|DONTFREE|CONVC|CONVO);

 	vp->tval |= STR;

 	vp->fmt = NULL;

-	setfree(vp);

 	DPRINTF("setsval %p: %s = \"%s (%p) \", t=%o r,f=%d,%d\n",

 		(void*)vp, NN(vp->nval), t, (void*)t, vp->tval, donerec, donefld);

 	vp->sval = t;

-- 

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