Fix instantiation and array accesses in class base_uint<BITS>

The implementation of base_uint::operator++(int) and base_uint::operator--(int) is now safer.
Array pn is accessed via index i after bounds checking has been performed on the index, rather than before.
The logic of the while loops has also been made more clear.

A compile time assertion has been added in the class constructors to ensure that BITS is a positive multiple of 32.

src/arith_uint256.cpp

@@ -15,6 +15,8 @@
 template <unsigned int BITS>
 base_uint<BITS>::base_uint(const std::string& str)
 {
+    static_assert(BITS/32 > 0 && BITS%32 == 0, "Template parameter BITS must be a positive multiple of 32.");
+
     SetHex(str);
 }