When length is 0, the code does nothing, so skip it completely.

Previously, os.Push(0) would do nothing as well. But with the newly added assertion, is the stack is empty, it will fail the assertion.

include/rapidjson/reader.h

@@ -948,11 +948,13 @@ private:
     #else
                 length = static_cast<SizeType>(__builtin_ffs(r) - 1);
     #endif
-                char* q = reinterpret_cast<char*>(os.Push(length));
-                for (size_t i = 0; i < length; i++)
-                    q[i] = p[i];
+                if (length != 0) {
+                    char* q = reinterpret_cast<char*>(os.Push(length));
+                    for (size_t i = 0; i < length; i++)
+                        q[i] = p[i];
 
-                p += length;
+                    p += length;
+                }
                 break;
             }
             _mm_storeu_si128(reinterpret_cast<__m128i *>(os.Push(16)), s);