[DFSan] Fix parameters to strtoull wrapper.

base and nptr_label were swapped, which meant we were passing nptr's
shadow as the base to the operation.  Usually, the shadow is 0, which
causes strtoull to guess the correct base from the string prefix (e.g.,
0x means base-16 and 0 means base-8), hiding this bug.  Adjust the test
case to expose the bug.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D85935