Fix inverse_discrete_quantile for large guess (#1007)

If `guess` passed to `inverse_discrete_quantile` cannot be represented
as floating point number, it is possible that `guess + 1` or `guess - 1`
does not change the value at all and we are stuck in infinite loop
inside `round_to_floor` or `round_to_ceil`.  Fix this by
increase/decrease more than 1 in these cases.

Example code to reproduce this:
```c++
boost::math::binomial_distribution<> dist(9079765771874083840, 0.561815);
boost::math::quantile(dist, 0.0365346);
```

Author: Yuhta

include/boost/math/distributions/detail/inv_discrete_quantile.hpp

@@ -302,15 +302,13 @@ inline typename Dist::value_type round_to_floor(const Dist& d, typename Dist::va
    //
    while(result != 0)
    {
-      cc = result - 1;
+      cc = floor(float_prior(result));
       if(cc < support(d).first)
          break;
       pp = c ? cdf(complement(d, cc)) : cdf(d, cc);
-      if(pp == p)
-         result = cc;
-      else if(c ? pp > p : pp < p)
+      if(c ? pp > p : pp < p)
          break;
-      result -= 1;
+      result = cc;
    }
 
    return result;
@@ -336,15 +334,13 @@ inline typename Dist::value_type round_to_ceil(const Dist& d, typename Dist::val
    //
    while(true)
    {
-      cc = result + 1;
+      cc = ceil(float_next(result));
       if(cc > support(d).second)
          break;
       pp = c ? cdf(complement(d, cc)) : cdf(d, cc);
-      if(pp == p)
-         result = cc;
-      else if(c ? pp < p : pp > p)
+      if(c ? pp < p : pp > p)
          break;
-      result += 1;
+      result = cc;
    }
 
    return result;

test/test_binomial.cpp

@@ -716,6 +716,18 @@ void test_spots(RealType T)
 
    check_out_of_range<boost::math::binomial_distribution<RealType> >(1, 1); // (All) valid constructor parameter values.
 
+   // TODO: Generic ibeta_power_terms has accuracy issue when long
+   // double is not precise enough, causing overflow in this case.
+   if(!std::is_same<RealType, real_concept>::value ||
+      sizeof(boost::math::concepts::real_concept_base_type) > 8)
+   {
+      using namespace boost::math::policies;
+      typedef policy<discrete_quantile<integer_round_outwards> > Policy;
+      binomial_distribution<RealType, Policy> dist(9079765771874083840, 0.561815);
+      // Accuracy is not too important here; the main purpose is to
+      // make sure it is not stuck.
+      BOOST_CHECK_CLOSE(quantile(dist, 0.0365346), 5101148604445670400, 1e12);
+   }
 
 } // template <class RealType>void test_spots(RealType)