Decryption functions can now accept a NULL pointer for the output

This checks the MAC without writing the decrypted message.

src/libsodium/crypto_aead/aes256gcm/aesni/aead_aes256gcm_aesni.c

@@ -787,9 +787,14 @@ crypto_aead_aes256gcm_decrypt_detached_afternm(unsigned char *m, unsigned char *
             d |= (mac[i] ^ (T[i] ^ accum[15 - i]));
         }
         if (d != 0) {
-            memset(m, 0, mlen);
+            if (m != NULL) {
+                memset(m, 0, mlen);
+            }
             return -1;
         }
+        if (m == NULL) {
+            return 0;
+        }
     }
     n2[3] = 0U;
     COUNTER_INC2(n2);

src/libsodium/crypto_aead/chacha20poly1305/sodium/aead_chacha20poly1305.c

@@ -198,6 +198,9 @@ crypto_aead_chacha20poly1305_decrypt_detached(unsigned char *m,
     (void) sizeof(int[sizeof computed_mac == 16U ? 1 : -1]);
     ret = crypto_verify_16(computed_mac, mac);
     sodium_memzero(computed_mac, sizeof computed_mac);
+    if (m == NULL) {
+        return ret;
+    }
     if (ret != 0) {
         memset(m, 0, mlen);
         return -1;
@@ -279,6 +282,9 @@ crypto_aead_chacha20poly1305_ietf_decrypt_detached(unsigned char *m,
     (void) sizeof(int[sizeof computed_mac == 16U ? 1 : -1]);
     ret = crypto_verify_16(computed_mac, mac);
     sodium_memzero(computed_mac, sizeof computed_mac);
+    if (m == NULL) {
+        return ret;
+    }
     if (ret != 0) {
         memset(m, 0, mlen);
         return -1;

src/libsodium/crypto_secretbox/crypto_secretbox_easy.c

@@ -96,6 +96,9 @@ crypto_secretbox_open_detached(unsigned char *m, const unsigned char *c,
         sodium_memzero(subkey, sizeof subkey);
         return -1;
     }
+    if (m == NULL) {
+        return 0;
+    }
     if (((uintptr_t) c >= (uintptr_t) m &&
          (uintptr_t) c - (uintptr_t) m < clen) ||
         ((uintptr_t) m >= (uintptr_t) c &&