Fix dangling pointer issue (#4786)

Fix dangling pointer issue (#4786)

clang may not realize that memcpy() doesn't keep the pointers handed
to it around, so there's no risk of the address of an auto being kept
past the caller's return, hence the "dangling pointer" warning.

Instead, head it off at the pass by immediately returning if len
is zero, so in needn't be set to the address of an auto; the only
explicit passing of NULL for in passes 0 for len. (Comments explain
how if len == 0, nothing changes in the context.) One could argue
for checking in rather than or in addition to checking len... OTOH,
shouldn't passing NULL and a non-zero len break loudly?