Fix NULL pointer passed to strlcpy from zap_lookup_impl()

Clang's static analyzer pointed out that whenever zap_lookup_by_dnode()
is called, we have the following stack where strlcpy() is passed a NULL
pointer for realname from zap_lookup_by_dnode():

strlcpy()
zap_lookup_impl()
zap_lookup_norm_by_dnode()
zap_lookup_by_dnode()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14044

Author: ryao

module/zfs/zap_micro.c

@@ -1013,8 +1013,10 @@ zap_lookup_impl(zap_t *zap, const char *name,
 			} else {
 				*(uint64_t *)buf =
 				    MZE_PHYS(zap, mze)->mze_value;
-				(void) strlcpy(realname,
-				    MZE_PHYS(zap, mze)->mze_name, rn_len);
+				if (realname != NULL)
+					(void) strlcpy(realname,
+					    MZE_PHYS(zap, mze)->mze_name,
+					    rn_len);
 				if (ncp) {
 					*ncp = mzap_normalization_conflict(zap,
 					    zn, mze);