fix(tsp): loop / crash if mmap of region fails

In test_memory_send the variable i is of unsigned type, so
it is never negative. If i is 0, the result of i-- is
4294967295. Don't know what happens if trying to
access composite->address_range_array[4294967295].
Made i a signed integer.

Signed-off-by: Thomas Viehweger <Thomas.Viehweger@rohde-schwarz.com>
Change-Id: I8b4e532749b5e86e4b5acd238e72c3f88e309ff2