[libc++] Explicitly cast in generate_canonical

A new clang warning introduced in r367497 was complaining about
the change in value.

Thanks to Brian Cain for the patch.
Differential Revision: https://reviews.llvm.org/D66422

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369393 91177308-0d34-0410-b5e6-96231b3b80d8

include/random

@@ -3645,7 +3645,7 @@ generate_canonical(_URNG& __g)
     const size_t __logR = __log2<uint64_t, _URNG::max() - _URNG::min() + uint64_t(1)>::value;
 #endif
     const size_t __k = __b / __logR + (__b % __logR != 0) + (__b == 0);
-    const _RealType _Rp = _URNG::max() - _URNG::min() + _RealType(1);
+    const _RealType _Rp = static_cast<_RealType>(_URNG::max() - _URNG::min()) + _RealType(1);
     _RealType __base = _Rp;
     _RealType _Sp = __g() - _URNG::min();
     for (size_t __i = 1; __i < __k; ++__i, __base *= _Rp)