CVE-2021-37692: Segfault on strings tensors with mistmatched dimensions, due to Go code

Under certain conditions, Go code can trigger a segfault in string deallocation.

For string tensors, C.TF_TString_Dealloc is called during garbage collection within a finalizer function. However, tensor structure isn’t checked until encoding to avoid a performance penalty. The current method for dealloc assumes that encoding succeeded, but segfaults when a string tensor is garbage collected whose encoding failed (e.g., due to mismatched dimensions).

To fix this, the call to set the finalizer function is deferred until NewTensor returns and, if encoding failed for a string tensor, deallocs are determined based on bytes written.