Memory leak on new StrRec

This commit: ee10850a Its causing string memory leaks on Linux AArch64. I have not checked other Arch's.

To test: program project1; var s:string; begin SetLength(S,100); end.

The above program leaks 117 bytes on Linux AArch64. If a previous commit (parent) is used for compilation (ea6529ff) no leaks occur.

Additional info. Lazarus trunk crashes due to out-of-memory errors when latest FPC trunk is used within a couple of minutes. Lazarus trunk runs perfect with low memory consumption when the parent of the offending FPC commit is used.