libfiemap: array boundary check for fiemap.fm_extents[]

Ensure we have at least one element before we try to access the last
element. Else the array index of the last element may underflow,
0ull - 1 == ~0ull == UINT64_MAX.

Bug: 204536075
Test: atest fiemap_writer_test
Change-Id: Ic390d108bf789cfe136fb5dfe2983f3c7d6f7e48