Additional check for Troe coefficients being zero

[g3bk47]

I encountered the problem that the "init" function of Troe reactions generates a floating point exception due to division by zero. I am using the FORCE Caltech mechanism from here:
http://www.theforce.caltech.edu/CaltechMech/
In the mechanism file
http://cvs.theforce.caltech.edu/cgi-bin/viewvc.cgi/CaltechMech/CaltechMech.chmech?view=co
there is the following reaction:

H+O2(+M) = HO2(+M) 5.590e+13 0.200 0.00
O2/0.89/ H2/1.50/ H2O/11.70/ CO2/2.18/ AR/0.52/ CO/1.09/
LOW / 2.650e+19 -1.300 0.00 /
TROE/ 0.7 0.00 10000000000.00 10000000000.00 /

The second Troe parameter is zero. According to the Chemkin Format
http://akrmys.com/public/chemkin/CKm_inp.html.en
this corresponds to T*** or "m_rt3" in Cantera:

cantera/src/kinetics/Falloff.cpp

Line 33 in 5c783c7

m_rt3 = 1.0/c[1];

Using the standard installation of Cantera, this does not generate any errors. But in my case, I use Cantera together with another program (OpenFOAM), which throws hardware exceptions if this occurs. Of course, I could change this by hand in the mechanism file itself, but maybe it is nicer to handle this in the code.
This was actually handled in previous versions, but was removed in this commit:
84e6775#diff-4210fe882743537c1bb1cacdbe10255b
and this issue:
#404
I would argue to include a special treatment for zero in case Cantera is used together with programs like OpenFOAM which will trigger hardware exceptions if division by zero occurs.

Because this part of the code is not relevant to performance, I introduced an additional check. This will prevent hardware exceptions in case c[1] or c[2] are zero but will exactly keep the current behaviour if c[1] and c[2] are not zero.

[codecov]

Codecov Report

Merging #601 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #601      +/-   ##
=========================================
+ Coverage    68.5%   68.5%   +<.01%     
=========================================
  Files         363     363              
  Lines       39957   39961       +4     
=========================================
+ Hits        27374   27377       +3     
- Misses      12583   12584       +1

Impacted Files	Coverage Δ
src/kinetics/Falloff.cpp	74.19% <100%> (+1.77%)	⬆️
src/transport/GasTransport.cpp	90.77% <0%> (-0.21%)	⬇️

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[codecov]

Codecov Report

Merging #601 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
+ Coverage   68.52%   68.52%   +<.01%     
==========================================
  Files         363      363              
  Lines       39953    39957       +4     
==========================================
+ Hits        27378    27381       +3     
- Misses      12575    12576       +1

Impacted Files	Coverage Δ
src/kinetics/Falloff.cpp	74.19% <100%> (+1.77%)	⬆️
src/transport/GasTransport.cpp	90.77% <0%> (-0.21%)	⬇️

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[speth]

Just pushed an updated version to fix some formatting issues (trailing whitespace and placement of curly braces). Good to merge once the CI completes.