9 dephects on prob_distr.c (April 2019)

Similar to legacy/trac#29805 (moved), coverity is again furious at prob distr:

We got a bunch of these:

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*** CID 1444030:  Incorrect expression  (SIZEOF_MISMATCH)
/src/test/test_prob_distr.c: 1254 in test_stochastic_weibull_impl()
1248     }
1249     
1250     static bool
1251     test_stochastic_weibull_impl(double lambda, double k)
1252     {
1253       const struct weibull dist = {
>>>     CID 1444030:  Incorrect expression  (SIZEOF_MISMATCH)
>>>     Adding "0UL /* 0 * sizeof (&dist - (struct weibull const *)&dist) */" to pointer "&weibull_ops" of type "struct dist_ops const *" is
>>> suspicious because adding an integral value to this pointer automatically scales that value by the size, 48 bytes, of the pointed-to type,
>>> "struct dist_ops const".  Most likely, the multiplication by "sizeof (0L)" in this expression is extraneous and should be eliminated.
1254         .base = WEIBULL(dist),
1255         .lambda = lambda,
1256         .k = k,
1257       };
1258     
1259     /*

and a bunch of these:

________________________________________________________________________________________________
*** CID 1415723:  Incorrect expression  (DIVIDE_BY_ZERO)
/src/feature/client/circpathbias.c: 194 in pathbias_get_scale_ratio()
188       (void) options;
189       /**
190        * The mult factor is the numerator for our scaling
191        * of circuit counts for our path bias window. It
192        * allows us to scale by fractions.
193        */
>>>     CID 1415723:  Incorrect expression  (DIVIDE_BY_ZERO)
>>>     In expression "networkstatus_get_param(NULL, "pb_multfactor", 1, 1, denominator) / (double)denominator", division by expression
>>> "denominator" which may be zero has undefined behavior.
194       return networkstatus_get_param(NULL, "pb_multfactor",
195                                   1, 1, denominator)/((double)denominator);
196     }
197     
198     /** The minimum number of circuit usage attempts before we start
199       * thinking about warning about path use bias and dropping guards */