Fixes issues our internal compliance and code quality tool found:

* Adds nullptr checks and asserts to protect certain code paths
* Fixes wrong integer type in a printf call
* Adds const to const values
* Prevents integer overflow with explicit casts

code/Common/DefaultIOSystem.cpp

@@ -173,7 +173,7 @@ inline static std::string MakeAbsolutePath(const char *in) {
         free(ret);
     }
 #endif
-    if (!ret) {
+    else {
         // preserve the input path, maybe someone else is able to fix
         // the path before it is accessed (e.g. our file system filter)
         ASSIMP_LOG_WARN("Invalid path: ", std::string(in));