DR crashes when analyzing UPX packed binaries on Ubuntu

DynamoRio crashes with Bus Error when instrumenting a binary packed with UPX (on ubuntu).

How to reproduce:
```bash
cat << EOF > hello.c
#include
int main(void) {
printf("hello world\n");
return 0;
}
EOF
gcc -static -o hello hello.c
upx -v -o hello_upx hello
drrun -- hello_upx
## Bus error (core dumped)
```

Platform: Ubuntu 16.04 LTS x86_64
Dr Version: drrun version 6.2.17231 -- build 0 (Commit: a7ff3fa3a70b98e42d49c8a7a6d016e82fa0c6da)
gcc: 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4)
UPX: 3.91