Merge #839: Prevent arithmetic on NULL pointer if the scratch space is too small

Merge #839: Prevent arithmetic on NULL pointer if the scratch space is too small

29a299e373d5f0e326be74c514c7c70ddf50cce1 Run the undefined behaviour sanitizer on Travis (Fabien)
7506e064d791e529d2e57bb52c156deb33b897ef Prevent arithmetic on NULL pointer if the scratch space is too small (Fabien)

Pull request description:

ACKs for top commit:
  sipa:
    ACK 29a299e373d5f0e326be74c514c7c70ddf50cce1. Reviewed the code changes and verified that building with these sanitizer flags catches the existing error, as well as a signed integer overflow if introduced.
  real-or-random:
    ACK 29a299e373d5f0e326be74c514c7c70ddf50cce1 code inspection
  jonasnick:
    utACK 29a299e373d5f0e326be74c514c7c70ddf50cce1

Tree-SHA512: 4d788f12f3d7b48018e884910adb9b530a05d88f504de83dadeab8a22d75da83c05a1518f7317de5f536c4dd243ea7b347b1eaddb2ca1d804c663e41b85db69d