heap-buffer-overflow in dpx::WriteBuffer

Hello.

I found a heap-buffer-overflow bug in oiio.

Please confirm.

Thanks.

OS: Ubuntu 17.04 64bit
Version: commit 5aef4903496c3c344f29613a3e8ce47a611f4774
PoC Download: [OV_dpx__BaseTypeConverter.zip](https://github.com/OpenImageIO/oiio/files/1688095/OV_dpx__BaseTypeConverter.zip)

Steps to reproduce:
1.Download the .POC files.
2.Compile the source code with ASan.
3.Execute the following command
: ./oiiotool $POC -o null.dpx

```
=================================================================
==5172==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62c0000085c0 at pc 0x7fd7ae3c40cc bp 0x7fff3f91c2a0 sp 0x7fff3f91c290
READ of size 2 at 0x62c0000085c0 thread T0
#0 0x7fd7ae3c40cb in dpx::BaseTypeConverter(unsigned short&, unsigned short&) /home/karas/oiio/src/dpx.imageio/libdpx/BaseTypeConverter.h:92
#1 0x7fd7ae3c40cb in void dpx::MultiTypeBufferCopy(unsigned short*, unsigned short*, int) /home/karas/oiio/src/dpx.imageio/libdpx/WriterInternal.h:72
#2 0x7fd7ae3c40cb in void dpx::CopyWriteBuffer(dpx::DataSize, unsigned char*, unsigned short*, int) /home/karas/oiio/src/dpx.imageio/libdpx/WriterInternal.h:82
#3 0x7fd7ae3c40cb in int dpx::WriteBuffer(OutStream*, dpx::DataSize, void*, unsigned int, unsigned int, int, dpx::Packing, bool, bool, int, char*, bool&, bool) /home/karas/oiio/src/dpx.imageio/libdpx/WriterInternal.h:292
#4 0x7fd7ae3bab3f in dpx::Writer::WriteElement(int, void*, dpx::DataSize) /home/karas/oiio/src/dpx.imageio/libdpx/Writer.cpp:316
#5 0x7fd7ae348fc5 in OpenImageIO_v1_9::DPXOutput::write_buffer() /home/karas/oiio/src/dpx.imageio/dpxoutput.cpp:556
#6 0x7fd7ae36643c in OpenImageIO_v1_9::DPXOutput::open(std::__cxx11::basic_string, std::allocator > const&, OpenImageIO_v1_9::ImageSpec const&, OpenImageIO_v1_9::ImageOutput::OpenMode) /home/karas/oiio/src/dpx.imageio/dpxoutput.cpp:196
#7 0x4f9574 in output_file /home/karas/oiio/src/oiiotool/oiiotool.cpp:4733
#8 0x7fd7adda2ef1 in OpenImageIO_v1_9::ArgOption::invoke_callback(int, char const**) const /home/karas/oiio/src/libutil/argparse.cpp:78
#9 0x7fd7adda2ef1 in OpenImageIO_v1_9::ArgParse::parse(int, char const**) /home/karas/oiio/src/libutil/argparse.cpp:379
#10 0x437419 in getargs /home/karas/oiio/src/oiiotool/oiiotool.cpp:5305
#11 0x42a6eb in main /home/karas/oiio/src/oiiotool/oiiotool.cpp:5526
#12 0x7fd7ab9e782f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
#13 0x42d8d8 in _start (/home/karas/oiio/run/tlr/usr/local/bin/oiiotool+0x42d8d8)

AddressSanitizer can not describe address in more detail (wild memory access suspected).
SUMMARY: AddressSanitizer: heap-buffer-overflow /home/karas/oiio/src/dpx.imageio/libdpx/BaseTypeConverter.h:92 dpx::BaseTypeConverter(unsigned short&, unsigned short&)
Shadow bytes around the buggy address:
0x0c587fff9060: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff9070: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff9080: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff9090: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff90a0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c587fff90b0: fa fa fa fa fa fa fa fa[fa]fa fa fa fa fa fa fa
0x0c587fff90c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff90d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff90e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff90f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c587fff9100: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Heap right redzone: fb
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack partial redzone: f4
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
==5172==ABORTING
```

=================
[Acknowledgement]
This work was supported by ICT R&D program of MSIP/IITP. [R7518-16-1001, Innovation hub for high Performance Computing]

Start with the ASan stack in src/dpx.imageio/libdpx/BaseTypeConverter.h:92 and trace the calls through WriterInternal.h, Writer.cpp, and dpxoutput.cpp. Build with ASan and run oiiotool using the linked PoC and -o null.dpx; done means the heap-buffer-overflow no longer occurs for this reproduction and coverage is added if the repository’s existing test location is identified.

Written by the indexing model from the issue text.