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dc: fix crashes with : operator (thanks istvan bak)

dc crashes because a Blk* sometimes ends getting double freed.
To make it crash, any of these lines will do:
(each line is a separate input to dc):

1 sa 2 :a le d sa v :a
1 sa 2 :a le d sa :a
1 sa 2 :a le d sa c

Fix by assigning p to sptr->val before EMTPY causes a jump.

Additionally, dcgetwd() can return 0. all other uses check for
0 ptr; Also fix a buffer overflow.

--- a/sys/src/cmd/dc.c

+++ b/sys/src/cmd/dc.c

@@ -638,8 +638,11 @@

 				p = sptr->val;

 				if(c >= ARRAYST) {

 					rewind(p);

-					while(sfeof(p) == 0)

-						release(dcgetwd(p));

+					while(sfeof(p) == 0) {

+						q = dcgetwd(p);

+						if(q != 0)

+							release(q);

+					}

 				}

 				release(p);

 			} else {

@@ -711,6 +714,7 @@

 					p = q;

 				}

 			}

+			sptr->val = p;

 			seekc(p,c*PTRSZ);

 			q = lookwd(p);

 			if(q!=0)

@@ -718,7 +722,6 @@

 			s = pop();

 			EMPTY;

 			salterwd(p, s);

-			sptr->val = p;

 			continue;

 		case ';':

 			p = pop();

@@ -1921,7 +1924,8 @@

 		sl = line;

 		*sl++ = c;

 		while((c = readc()) != '\n')

-			*sl++ = c;

+			if(sl-line < sizeof(line)-1)

+				*sl++ = c;

 		*sl = 0;

 		if((pid = fork()) == 0) {

 			execl("/bin/rc","rc","-c",line,nil);

-- 

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