Fix 'Dead nested assignment's from scan-build-10

The 3 warnings reported are:

os.c:872:7: warning: Although the value stored to 'ptr' is used in the enclosing expression, the value is never actually read from 'ptr'
        if ((ptr = strtok_r(command, " \t", &last)) == NULL) {
             ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

--

rpz.c:1117:10: warning: Although the value stored to 'zbits' is used in the enclosing expression, the value is never actually read from 'zbits'
        return (zbits &= x);
                ^        ~
1 warning generated.

--

openssleddsa_link.c:532:10: warning: Although the value stored to 'err' is used in the enclosing expression, the value is never actually read from 'err'
        while ((err = ERR_get_error()) != 0) {
                ^     ~~~~~~~~~~~~~~~
1 warning generated.

(cherry picked from commit 262f087bcf)

lib/dns/openssleddsa_link.c

@@ -518,7 +518,6 @@ static bool
 openssleddsa_isprivate(const dst_key_t *key) {
 	EVP_PKEY *pkey = key->keydata.pkey;
 	int len;
-	unsigned long err;
 
 	if (pkey == NULL) {
 		return (false);
@@ -529,7 +528,7 @@ openssleddsa_isprivate(const dst_key_t *key) {
 		return (true);
 	}
 	/* can check if first error is EC_R_INVALID_PRIVATE_KEY */
-	while ((err = ERR_get_error()) != 0) {
+	while (ERR_get_error() != 0) {
 		/**/
 	}
 

lib/dns/rpz.c

@@ -1114,7 +1114,8 @@ trim_zbits(dns_rpz_zbits_t zbits, dns_rpz_zbits_t found) {
 	x = zbits & found;
 	x &= (~x + 1);
 	x = (x << 1) - 1;
-	return (zbits &= x);
+	zbits &= x;
+	return (zbits);
 }
 
 /*