fix 1. Stack array case (WOLFSSL_SMALL_STACK not defined): buffer is byte buffer[WC_MAX_DIGEST_SIZE] - always valid, never NULL

  2. Heap case (WOLFSSL_SMALL_STACK defined): If XMALLOC fails, WC_ALLOC_VAR_EX returns MEMORY_E immediately, so we never reach ForceZero with a NULL pointer

Author: aidangarske

wolfcrypt/src/pwdbased.c

@@ -296,9 +296,7 @@ int wc_PBKDF2_ex(byte* output, const byte* passwd, int pLen, const byte* salt,
         wc_HmacFree(hmac);
     }
 
-    if (buffer != NULL) {
-        ForceZero(buffer, (word32)hLen);
-    }
+    ForceZero(buffer, (word32)hLen);
     WC_FREE_VAR_EX(buffer, heap, DYNAMIC_TYPE_TMP_BUFFER);
     WC_FREE_VAR_EX(hmac, heap, DYNAMIC_TYPE_HMAC);