Do not copy an empty run into an unallocated arena

memcpy() must not be passed a null pointer even for zero bytes, which
arena_run() did for an empty run while the arena was still empty
(ci_test_clang_sanitizer: "null pointer passed as argument 1" in
builder.hpp). Only the slow path can see such a run.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json_view.hpp

@@ -1039,8 +1039,10 @@ class builder
             arena_ensure(16);
             std::memcpy(m_arena_write, r, 16);
         }
-        else
+        else if (n != 0)
         {
+            // (an empty run leaves an empty arena unallocated, and memcpy()
+            // must not be passed its null pointer even for zero bytes)
             arena_ensure(n);
             std::memcpy(m_arena_write, r, n);
         }