NULL deref SEGV in malloc.c:unbin()

[David Wuertele <dave+gmane@wuertele.com>]

I wonder if anyone has hit this before?   In unbin(), c->next->prev is set, but
c->next is NULL.   It happens repeatedly, and here's what gdb says:

(gdb) b fopen
Breakpoint 9 at 0x90f78: file src/stdio/fopen.c, line 13.
(gdb) c
Continuing.

Breakpoint 9, fopen (filename=0xaabe4 "/etc/hosts", mode=0xaabf0 "r")
    at src/stdio/fopen.c:13
13	src/stdio/fopen.c: No such file or directory.
	in src/stdio/fopen.c
(gdb) b unbin
Breakpoint 10 at 0x8bc44: file src/malloc/malloc.c, line 239.
(gdb) c
Continuing.

Breakpoint 10, unbin (c=0x21408b8, i=40) at src/malloc/malloc.c:239
239	src/malloc/malloc.c: No such file or directory.
	in src/malloc/malloc.c
(gdb) print *c
$6 = {psize = 2096, csize = 2097, next = 0x2140088, prev = 0x0}
(gdb) s
241	in src/malloc/malloc.c
(gdb) 
Program received signal SIGSEGV, Segmentation fault.
0x0008bcc0 in unbin (c=0x21408b8, i=40) at src/malloc/malloc.c:241
241	in src/malloc/malloc.c
(gdb) 

The root cause was not obvious on scanning the source.
Is this perhaps something that's already been fixed?

Dave