Add assert for possible buffer overrun.

I don't know if this can happen, but better to put assert to detect it.

C6386 Buffer overrun while writing to 'np':  the writable size is 'm_size*m_size*3*4' bytes, but '16' bytes might be written.
line 213: 'this->m_normals' may be NULL (Enter this branch)
line 217: (alias) 'np' receives the value of 'this->m_normals'
line 238: Skip this loop, (assume 'i<m_res' is false)
line 255: Skip this loop, (assume 'j<m_res' is false)
line 277: Invalid write to 'np[3]', (writable range is 0 to 2)
Check for nullptr.

Based on static analyzer it can be nullptr.
Fix Android.