Fix soinfo_tls::module dangling reference

The field was pointing into an element of an std::vector, but the address
of a vector element is invalidated when the vector is resized.

This bug was caught by the new elftls.shared_ie and
elftls_dl.dlopen_shared_var_ie tests.

Bug: http://b/78026329
Test: bionic unit tests
Change-Id: I7232f6d703a9e339fe8966a95b7a68bae2c9c420