Fix 411 div by zero in ctgsja

[weslleyspereira]

Closes #411

This PR replaces A1.NE.ZERO with GAMMA is finite in the subroutines xTGSJA.

[codecov]

Codecov Report

Merging #502 (177938c) into master (d71dd6e) will decrease coverage by 0.08%.
The diff coverage is 86.44%.

❗ Current head 177938c differs from pull request most recent head 5a7bde2. Consider uploading reports for the commit 5a7bde2 to get more accurate results

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
- Coverage   83.41%   83.32%   -0.09%     
==========================================
  Files        1821     1820       -1     
  Lines      185725   170881   -14844     
==========================================
- Hits       154914   142393   -12521     
+ Misses      30811    28488    -2323     

Impacted Files	Coverage Δ
SRC/stgsja.f	91.33% <85.71%> (-1.90%)	⬇️
SRC/ctgsja.f	91.72% <86.66%> (-1.75%)	⬇️
SRC/dtgsja.f	91.33% <86.66%> (-1.90%)	⬇️
SRC/ztgsja.f	91.72% <86.66%> (-1.75%)	⬇️
SRC/zggsvd3.f	89.70% <0.00%> (-7.36%)	⬇️
SRC/dlaqr1.f	47.61% <0.00%> (-6.23%)	⬇️
SRC/slaqr1.f	47.61% <0.00%> (-6.23%)	⬇️
SRC/dlals0.f	74.73% <0.00%> (-4.76%)	⬇️
SRC/slals0.f	74.73% <0.00%> (-4.76%)	⬇️
SRC/chegst.f	86.44% <0.00%> (-4.58%)	⬇️
... and 1819 more

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[langou]

[ reverting my comment ]

[weslleyspereira]

Using PR #493 and the test case from #411 (comment), we obtain a NaN in the output although INFO=0:

 INFO =            0
 K =            0
 L =            6
 ALPHA:    2.73536585E-15   1.48088933E-27   1.57898426E-33   0.00000000       0.00000000       0.00000000    
 BETA:     1.00000000       1.00000000       1.00000000                  NaN   1.00000000       1.00000000

3. With the solution from this PR (Fix 411 div by zero in ctgsja #502), I obtain:

 INFO =            0
 K =            0
 L =            6
 ALPHA:    2.73536606E-15   1.48088913E-27   1.57898444E-33   0.00000000       0.00000000       0.00000000    
 BETA:     1.00000000       1.00000000       1.00000000       1.00000000       1.00000000       1.00000000

My last commit tries to improve the readability of my changes.