Fix comparison bw integer and pointer (#4155)
Fix comparison bw integer and pointer (#4155)

* Fix comparison bw integer and pointer

Error: ISO C++ forbids comparison between pointer and integer [-fpermissive]

* Check for nullptr input string.

platsch/Slic3r,tmotl/Slic3r,alexrj/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,lordofhyphens/Slic3r,platsch/Slic3r,lordofhyphens/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,tmotl/Slic3r,platsch/Slic3r,tmotl/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,xoan/Slic3r,xoan/Slic3r,curieos/Slic3r,curieos/Slic3r,platsch/Slic3r,tmotl/Slic3r,xoan/Slic3r,pieis2pi/Slic3r,xoan/Slic3r,xoan/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,xoan/Slic3r,tmotl/Slic3r,curieos/Slic3r,alexrj/Slic3r,platsch/Slic3r,pieis2pi/Slic3r,alexrj/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,tmotl/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,curieos/Slic3r,platsch/Slic3r,lordofhyphens/Slic3r