input_buffer_adapter: Fix handling of nullptr input

Clang UBSAN currently complains that the char * to input_buffer_adapter
is a nullptr.

Turns out it is actually required to accept nullptr, see for example
line 415 in input_adapters.hpp

  ...
  // the address of first cannot be used: use nullptr
  ia = std::make_shared<input_buffer_adapter>(nullptr, len);
  ....

Therefore we have to handle it gracefully here. We now also ignore the
length parameter l if b is a nullptr.

Author: t-b

include/nlohmann/detail/input/input_adapters.hpp

@@ -131,9 +131,8 @@ class input_stream_adapter : public input_adapter_protocol
 class input_buffer_adapter : public input_adapter_protocol
 {
   public:
-    JSON_HEDLEY_NON_NULL(2)
     input_buffer_adapter(const char* b, const std::size_t l) noexcept
-        : cursor(b), limit(b + l)
+        : cursor(b), limit(b == nullptr ? nullptr : (b + l))
     {}
 
     // delete because of pointer members
@@ -147,6 +146,7 @@ class input_buffer_adapter : public input_adapter_protocol
     {
         if (JSON_HEDLEY_LIKELY(cursor < limit))
         {
+            assert(cursor != nullptr && limit != nullptr);
             return std::char_traits<char>::to_int_type(*(cursor++));
         }