[metadata] Fix a possible crash when looking up a p/invoke function. (#4471)

[metadata] Fix a possible crash when looking up a p/invoke function. (#4471)

It's possible for two threads to modify the same MonoMethodPInvoke.addr
in mono_lookup_pinvoke_call, with one thread setting it to NULL inside the
loop, and the other thread having completed the loop and expecting it to
be non-NULL.

Use an intermediate variable to prevent addr becoming non-NULL.