Address issues found by ubsan

Running system tests after building BIND using Clang 6.0 with -fsanitize=undefined yields a few benign warnings:

rdataslab.c:84:33: runtime error: member access within address <address> with insufficient space for an object of type 'const struct xrdata'`

client.c:2625:4: runtime error: null pointer passed as argument 2, which is declared to never be null`

buffer.c:62:22: runtime error: null pointer passed as argument 2, which is declared to never be null`