Segfault in libc -- call to malloc from libpng

[Jim Ziegler <jcz@ziplink.net>]

I have a program which uses libpng.  Libpng makes several
calls to malloc.  At some point libc segfaults.

Stack trace:
    0 ... in free      from libc
    1 ... in malloc    from libc
    2 ... in malloc    from libc
    3 ... in pngmalloc at pngmem:405

Code at pngmalloc:pngmem:405
    ret = malloc((size_t)size);

Contents of size: 1729

Any suggestions?