Fix leaking definitions on FFI::cdef()->new()

Previously, FFI_G(symbols) and FFI_G(tags) were never cleaned up when calling
new on an existing object. However, if cdef() is called withou

Fix leaking definitions on FFI::cdef()->new()

Previously, FFI_G(symbols) and FFI_G(tags) were never cleaned up when calling
new on an existing object. However, if cdef() is called without parameters these
globals are NULL and might be created when new() creates new definitions. These
would then be discarded without freeing them.

Closes GH-11751

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