Argon2: initialize ctx{.pwd,.pwdlen} in the verify function

Keep initializing the length for clarity;
Compilers know how to optimize this out.

src/libsodium/crypto_pwhash/argon2/argon2-encoding.c

@@ -299,7 +299,6 @@ int decode_string(argon2_context *ctx, const char *str, argon2_type type) {
     ctx->adlen = 0;
     ctx->saltlen = 0;
     ctx->outlen = 0;
-    ctx->pwdlen = 0;
     if (type == Argon2_i) {
         CC("$argon2i");
     } else {

src/libsodium/crypto_pwhash/argon2/argon2.c

@@ -183,6 +183,8 @@ int argon2_verify(const char *encoded, const void *pwd, const size_t pwdlen,
 
     memset(&ctx, 0, sizeof ctx);
 
+    ctx.pwd = NULL;
+    ctx.pwdlen = 0;
     ctx.secret = NULL;
     ctx.secretlen = 0;