Post-push fix. Some callers of dynstr_append_mem() apparently depend
on the side-effects of zero-termination when appending a zero-sized string.
This is from clang version 6.0.0-1ubuntu2 with ASAN:

==25852==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x604000189d40 at pc 0x0000004e076c bp 0x7ffd77220550 sp 0x7ffd7721fd00
READ of size 17 at 0x604000189d40 thread T0
    #0 0x4e076b in __interceptor_strlen.part.30 asan_interceptors.cc.o:?
    #1 0x5b0496 in init_dynamic_string(DYNAMIC_STRING*, char const*, unsigned long, unsigned long) mysys/my_string.cc:51
    #2 0x56accd in do_connect(st_command*) client/mysqltest.cc:5970
    #3 0x5680c9 in main client/mysqltest.cc:8945
    #4 0x7f027e87db96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310

The fix is to skip memcpy() only, if length == zero, rather than
skipping the entire function.

Change-Id: I10540d7c87ae1cfed3bc6967e42290401f5d1f8d