isc_buffer_*(): if source can be NULL, only call memmove() when length is non-zero

Certain isc_buffer_*() functions might call memmove() with the second
argument (source) set to NULL and the third argument (length) set to 0.
While harmless, it triggers an ubsan warning:

    runtime error: null pointer passed as argument 2, which is declared to never be null

Modify all memmove() call sites in lib/isc/include/isc/buffer.h and
lib/isc/buffer.c which may potentially use NULL as the second argument
(source) so that memmove() is only called if the third argument (length)
is non-zero.

(cherry picked from commit 6ddbca6f)
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