Fixing warnings about conversions in GCC 7+ (#1753)

Author: henryiii

include/pybind11/numpy.h

@@ -855,14 +855,14 @@ template <typename T, int ExtraFlags = array::forcecast> class array_t : public
 
     // Reference to element at a given index
     template<typename... Ix> const T& at(Ix... index) const {
-        if (sizeof...(index) != ndim())
+        if ((ssize_t) sizeof...(index) != ndim())
             fail_dim_check(sizeof...(index), "index dimension mismatch");
         return *(static_cast<const T*>(array::data()) + byte_offset(ssize_t(index)...) / itemsize());
     }
 
     // Mutable reference to element at a given index
     template<typename... Ix> T& mutable_at(Ix... index) {
-        if (sizeof...(index) != ndim())
+        if ((ssize_t) sizeof...(index) != ndim())
             fail_dim_check(sizeof...(index), "index dimension mismatch");
         return *(static_cast<T*>(array::mutable_data()) + byte_offset(ssize_t(index)...) / itemsize());
     }