memory.cc: fix cross-shard shrinking realloc

[travisdowns]

Cross-shard shrinking realloc crashes because we assert that the incoming pointer is shard local inside the shrink method but there is no particular reason to assume this is the case with a realloc: the initial allocation may have been made on another shard.

Fix this by falling through to the free/malloc/memcpy path. This also means that realloc using the same size is a way to "rehome" a possibly foreign pointer: this does nothing if the pointer is already local but it will allocate a local pointer and copy the allocation contents if not.

Fixes #2202.