i.smap: fix possible pole error with log in extract function

Using logarithm function call with zero argument will lead to
a pole error, which occurs if the mathematical function has
an exact infinite result.

Check if the argument value is zero before passing that to
the log function to avoid such errors. I also added check
for negative numbers just to make sure the argument is in
the right domain for the log function as well.

Signed-off-by: Mohan Yelugoti <ymdatta.work@gmail.com>

imagery/i.smap/model.c

@@ -155,7 +155,8 @@ void extract(DCELL ***img,          /* multispectral image, img[band][i][j] */
                         for (k = 0; k < C->nsubclasses; k++)
                             subsum += exp(subll[k] - maxlike) * C->SubSig[k].pi;
 
-                        ll[i][j][m] = log(subsum) + maxlike;
+                        if (subsum > 0)
+                            ll[i][j][m] = log(subsum) + maxlike;
                     }
                 }
             }