Don't fall through between cases in light_mem_seek()

[IsaacsLab42]

The switch statement in the function light_mem_seek() falls through all of the cases, resulting in an invalid offset, as described in issue #32.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 74.25%. Comparing base (f595e4b) to head (1609023).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/light_io_mem.c	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   74.49%   74.25%   -0.24%     
==========================================
  Files          17       17              
  Lines        1145     1146       +1     
  Branches        0      151     +151     
==========================================
- Hits          853      851       -2     
- Misses        292      295       +3     

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