cmd/ld: another use-after-free

This only shows up in the duplicate symbol error message.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7486053

src/cmd/5l/obj.c

@@ -428,6 +428,7 @@ ldobj1(Biobuf *f, char *pkg, int64 len, char *pn)
 	ntext = 0;
 	eof = Boffset(f) + len;
 	src[0] = 0;
+	pn = estrdup(pn); // we keep it in Sym* references
 
 newloop:
 	memset(h, 0, sizeof(h));

src/cmd/6l/obj.c

@@ -423,6 +423,7 @@ ldobj1(Biobuf *f, char *pkg, int64 len, char *pn)
 	ntext = 0;
 	eof = Boffset(f) + len;
 	src[0] = 0;
+	pn = estrdup(pn); // we keep it in Sym* references
 
 newloop:
 	memset(h, 0, sizeof(h));

src/cmd/8l/obj.c

@@ -444,7 +444,7 @@ ldobj1(Biobuf *f, char *pkg, int64 len, char *pn)
 	ntext = 0;
 	eof = Boffset(f) + len;
 	src[0] = 0;
-
+	pn = estrdup(pn); // we keep it in Sym* references
 
 newloop:
 	memset(h, 0, sizeof(h));