BUG: fitpack: avoid a memory error in splev(x, tck, der=k)

for x < t[0], splev(..., der=k) produces garbage or crashes
because of a memory error.

Fixes gh-2188
Merge pull request #4093 from ev-br/splev_der_k

BUG: fitpack: avoid a memory error in splev(x, tck, der=k)