fix float point overflow bug

[vliqi]

beacause of float point overflow, run SST turbulence model on
single precision version cfl3d program may cause crash problem.
such as On ONERA M6 WING Test Case (https://cfl3d.larc.nasa.gov/Cfl3dv6/cfl3dv6_testcases.html#onera),
if change ivisc=5 to 7, even set edvislim keyword to 100000, the program will cause crash error:

`Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.

Backtrace for this error:
#0 0x7f4503de6d4a
#1 0x7f4503de5f7d
#2 0x7f450330502f
#3 0x563ac206f6e4
#4 0x563ac2322f71
#5 0x563ac21f987d
#6 0x563ac23a5a5d
#7 0x563ac23b45f8
#8 0x563ac24077b4
#9 0x563ac2054048
#10 0x7f45032f22b0
#11 0x563ac2054079
#12 0xffffffffffffffff`

[crumsey]

We have to be careful with this kind of fix. It is changing the model, albeit apparently only in a minor way. But it is a change. Old results will no longer be duplicated.