Problem reported upstream at
http://www.zsh.org/mla/users/2016/msg00710.html

  A possible patch has been reported upstream as
well:

---8<---
$ gendiff zsh-5.0.2 .orig
diff -up zsh-5.0.2/Src/exec.c.orig zsh-5.0.2/Src/exec.c
--- zsh-5.0.2/Src/exec.c.orig	2016-09-06 15:10:19.394565181 -0300
+++ zsh-5.0.2/Src/exec.c	2016-09-06 15:10:36.300551444 -0300
@@ -3631,16 +3631,16 @@ gethere(char **strp, int typ)
 	*bptr++ = '\n';
     }
     *t = '\0';
+    s = dupstring(buf);
+    zfree(buf, bsiz);
     if (!qt) {
 	int ef = errflag;
 
-	parsestr(buf);
+	parsestr(s);
 
 	if (!errflag)
 	    errflag = ef;
     }
-    s = dupstring(buf);
-    zfree(buf, bsiz);
     return s;
 }
 
---8<---

  The problem is caused by mixing system malloc data
and internal zsh allocator. It does not find the Heap
associated with the pointer passed to hrealloc and
crashes when dereferencing a null pointer.

  Simple reproducer:
---8<---
#!/usr/bin/zsh

cat >> /tmp/try <<EOF
export A="$(tr '\n' ' ' <<BLDARC
content
BLDARC)"
EOF
---8<---