Fix null pointer dereference in FilteredRE2::RegisterDynamic

[wangziqi520]

In re2/filtered_re2.cc, the function RegisterDynamic writes to the output parameter id without first checking whether it is nullptr. If a caller passes nullptr, this leads to undefined behavior and can crash the program.

This change adds a null check for id. If id is null, the function returns false early to avoid dereferencing it.

This fixes a potential crash when using FilteredRE2 with invalid arguments.

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