Fix potential buffer overflow in strcpy use on macOS

The null byte wasn't taken into account when allocating memory
to strcpy into.

The calculation to check if allocation was needed  was also wrong,
causing allocation for every string.

In practice it's not so likely users would ever hit this since
the function tended to over allocate, even in the case an off by one
error occurred, in all likelihood the room would already be available.

Ref !114512

source/blender/gpu/metal/mtl_shader_generator.mm

@@ -3480,8 +3480,8 @@ static uint32_t name_buffer_copystr(char **name_buffer_ptr,
   BLI_assert(ret_len > 0);
 
   /* If required name buffer size is larger, increase by at least 128 bytes. */
-  if (name_buffer_size + ret_len > name_buffer_size) {
-    name_buffer_size = name_buffer_size + max_ii(128, ret_len);
+  if (name_buffer_offset + ret_len + 1 > name_buffer_size) {
+    name_buffer_size = name_buffer_offset + max_ii(128, ret_len + 1);
     *name_buffer_ptr = (char *)MEM_reallocN(*name_buffer_ptr, name_buffer_size);
   }