fix memleak

[casperdcl]

span1_span11 does mallocs but the data was never freed.

[codecov]

Codecov Report

Merging #39 (6161467) into devel (4a05c82) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            devel      #39   +/-   ##
=======================================
  Coverage   14.50%   14.50%           
=======================================
  Files          16       16           
  Lines        2965     2965           
=======================================
  Hits          430      430           
  Misses       2535     2535           

Impacted Files	Coverage Δ
niftypet/nipet/__init__.py	100.00% <ø> (ø)
niftypet/nipet/mmraux.py	36.84% <ø> (ø)

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