The generic RC4 implementation may read and write just past the end of the
buffer; when input and output are aligned, it always reads an RC4_CHUNK at a
time. It appropriately masks off and preserves the excess bytes off the end, so
this can only have practical effects if it crosses a page boundary. There's an
alignment check, so that can't happen; page boundaries are always aligned. But
it makes ASan unhappy and strictly speaking is a memory error.

Instead, fall through to the generic codepath which just reads it byte by byte.
This should fix the other bot failure.

Change-Id: I3cbd3bfc6cb0537e87f3252dea12d40ffa78d590
Reviewed-on: https://boringssl-review.googlesource.com/4722
Reviewed-by: Adam Langley <agl@google.com>