Fixed a small issue with Custom Mem handler and MexFreeBlock calculation

src/custommem.c

@@ -92,7 +92,9 @@ static int getMaxFreeBlock(void* block, size_t block_size)
     int maxsize = 0;
     while(m->prev.x32) {    // while there is a subblock
         if(!m->prev.fill && m->prev.size>maxsize) {
-            maxsize = m->prev.size-sizeof(blockmark_t); // remove the marker
+            maxsize = m->prev.size;
+            if(maxsize>sizeof(blockmark_t))  // remove the marker
+                maxsize -= sizeof(blockmark_t);
             if((uintptr_t)block+maxsize>(uintptr_t)m)
                 return maxsize; // no block large enough left...
         }