tools/insn-fuzz: Don't hit memcpy() for zero-length reads

tools/insn-fuzz: Don't hit memcpy() for zero-length reads

For control-flow changes, the emulator needs to perform a zero-length
instruction fetch at the target offset.  It also passes NULL for the
destination buffer, as there is no instruction stream to collect.

This trips up UBSAN when passed to memcpy(), as passing NULL is undefined
behaviour per the C spec (irrespective of passing a size of 0).

Special case these fetches in fuzz_insn_fetch() before reaching data_read().

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>