Fixes CVE-2025-2750: out of bounds write by assigning to wrong array element count tracking (closes #6011) (#6225)

description:
- The current implementation has faulty reallocation logic when parsing a CSM file
- Issue 1
    - https://github.com/assimp/assimp/blob/4ad1d2aa3086517816716a50aa122342806736f9/code/AssetLib/CSM/CSMLoader.cpp#L205
    - By assigning s->mNumPositionKeys = alloc*2 right before resizing the buffer, making s->mNumPositionKeys equivalent to the
      max number of aiVectorKey that can be stored in s->mPositionKeys
    - the code later attempts to get the next write location by doing: aiVectorKey* sub = s->mPositionKeys + s->mNumPositionKeys;
    - this points to the end of the array, not after the last element in the array
- Issue 2
    - https://github.com/assimp/assimp/blob/4ad1d2aa3086517816716a50aa122342806736f9/code/AssetLib/CSM/CSMLoader.cpp#L178-L184
    - if the CSM file does not declare last frame data, then mPositionKeys will never be initialized

fix:
- we preserve s->mNumPositionKeys to still contain the actual number of aiVectorKeys and ensure that we will not write out of bounds
- we initialize mPositionKeys with a default value and if we find last frame info, we just re-initialize it

Co-authored-by: Vinz Spring <vinzs@amazon.de>
Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

code/AssetLib/CSM/CSMLoader.cpp

@@ -176,9 +176,17 @@ void CSMImporter::InternReadFile( const std::string& pFile,
                 // If we know how many frames we'll read, we can preallocate some storage
                 unsigned int alloc = 100;
                 if (last != 0x00ffffff) {
+                    // re-init if the file has last frame data
                     alloc = last-first;
                     alloc += alloc>>2u; // + 25%
                     for (unsigned int i = 0; i < anim->mNumChannels; ++i) {
+                        if (anim->mChannels[i]->mPositionKeys != nullptr) delete[] anim->mChannels[i]->mPositionKeys;
+                        anim->mChannels[i]->mPositionKeys = new aiVectorKey[alloc];
+                    }
+                } else {
+                    // default init
+                    for (unsigned int i = 0; i < anim->mNumChannels; ++i) {
+                        if (anim->mChannels[i]->mPositionKeys != nullptr) continue;
                         anim->mChannels[i]->mPositionKeys = new aiVectorKey[alloc];
                     }
                 }
@@ -202,7 +210,7 @@ void CSMImporter::InternReadFile( const std::string& pFile,
                         if (s->mNumPositionKeys == alloc)   {
                             // need to reallocate?
                             aiVectorKey* old = s->mPositionKeys;
-                            s->mPositionKeys = new aiVectorKey[s->mNumPositionKeys = alloc*2];
+                            s->mPositionKeys = new aiVectorKey[alloc*2];
                             ::memcpy(s->mPositionKeys,old,sizeof(aiVectorKey)*alloc);
                             delete[] old;
                         }