[axl_sl] added an assert to StringRef::attach (length must not be -1)
[axl_sl] added an assert to StringRef::attach (length must not be -1)

added after chasing an non-obvious propagation of -1 with subsequent modifications due to substring extraction

vovkos/axl,vovkos/axl,vovkos/axl,vovkos/axl