Dr. Memory always reports "write to readonly" as an app error

This error occurred inside the Dr. Memory fuzzer, but the error report makes it look like an app error:

```
Error #3: REPORTED WARNING: writing to readonly memory 0x0a471214-0x0a471218
#0 libdynamorio.so.5.0!memcpy_mid [/usr/local/google/home/byronh/work/
dr2/drmemory/dynamorio/core/lib/dr_helper.c:44]
(0xf713944c ) modid:3
Note: @0:00:06.949 in thread 1364
Note: instruction: rep movs %ds:(%esi) %esi %edi %ecx -> %es:(%edi) %esi %edi %ecx
error end
```