fix valgrind call and uninitialized values

[mmitch]

• Fix valgrind to check the gbsplay executable instead of make. Achieved via TEST_WRAPPER Makefile variable.
• Valgrind then reported two usages of uninitialized values which could both be fixed by changing the surrounding code to remove the affected code flow branches.

Remaining valgrind warnings:

==8848== Warning: ignored attempt to set SIGSTOP handler in sigaction();
==8848==          the SIGSTOP signal is uncatchable
==8848== 
==8848== HEAP SUMMARY:
==8848==     in use at exit: 8,400 bytes in 2 blocks
==8848==   total heap usage: 282 allocs, 280 frees, 123,965 bytes allocated

[codecov[bot]]

Codecov Report

Merging #53 (d566b4e) into master (18f2201) will decrease coverage by 0.18%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
- Coverage   45.31%   45.12%   -0.19%     
==========================================
  Files          19       19              
  Lines        2827     2830       +3     
==========================================
- Hits         1281     1277       -4     
- Misses       1546     1553       +7     

Impacted Files	Coverage Δ
terminal_posix.c	54.54% <66.66%> (-15.46%)	:arrow_down:
gbsplay.c	39.62% <0.00%> (-0.95%)	:arrow_down:

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