fix: Properly initialize RNG context

[AnthonyGrondin]

Fixes an issue where using some ciphers would cause Mbedtls to fail with MBEDTLS_ERR_SSL_NO_RNG.

I found this issue, while trying to save some space, by building with the following options, and running async_server.rs:

diff --git a/esp-mbedtls-sys/headers/esp32s3/config.h b/esp-mbedtls-sys/headers/esp32s3/config.h
index 72e4bba..3b94884 100644
--- a/esp-mbedtls-sys/headers/esp32s3/config.h
+++ b/esp-mbedtls-sys/headers/esp32s3/config.h
@@ -656,17 +656,17 @@
 #define MBEDTLS_ECP_DP_SECP192R1_ENABLED
 #define MBEDTLS_ECP_DP_SECP224R1_ENABLED
 #define MBEDTLS_ECP_DP_SECP256R1_ENABLED
-#define MBEDTLS_ECP_DP_SECP384R1_ENABLED
-#define MBEDTLS_ECP_DP_SECP521R1_ENABLED
+// #define MBEDTLS_ECP_DP_SECP384R1_ENABLED
+// #define MBEDTLS_ECP_DP_SECP521R1_ENABLED
 #define MBEDTLS_ECP_DP_SECP192K1_ENABLED
 #define MBEDTLS_ECP_DP_SECP224K1_ENABLED
 #define MBEDTLS_ECP_DP_SECP256K1_ENABLED
 #define MBEDTLS_ECP_DP_BP256R1_ENABLED
-#define MBEDTLS_ECP_DP_BP384R1_ENABLED
-#define MBEDTLS_ECP_DP_BP512R1_ENABLED
+// #define MBEDTLS_ECP_DP_BP384R1_ENABLED
+// #define MBEDTLS_ECP_DP_BP512R1_ENABLED
 /* Montgomery curves (supporting ECP) */
-#define MBEDTLS_ECP_DP_CURVE25519_ENABLED
-#define MBEDTLS_ECP_DP_CURVE448_ENABLED
+// #define MBEDTLS_ECP_DP_CURVE25519_ENABLED
+// #define MBEDTLS_ECP_DP_CURVE448_ENABLED

 /**
  * \def MBEDTLS_ECP_NIST_OPTIM
@@ -1516,7 +1516,7 @@
  * Comment this macro to disable storing the peer's certificate
  * after the handshake.
  */
-#define MBEDTLS_SSL_KEEP_PEER_CERTIFICATE
+// #define MBEDTLS_SSL_KEEP_PEER_CERTIFICATE

 /**
  * \def MBEDTLS_SSL_RENEGOTIATION
@@ -1605,7 +1605,7 @@
  *
  * Uncomment this macro to enable the support for TLS 1.3.
  */
-#define MBEDTLS_SSL_PROTO_TLS1_3
+// #define MBEDTLS_SSL_PROTO_TLS1_3

 /**
  * \def MBEDTLS_SSL_TLS1_3_COMPATIBILITY_MODE
@@ -1840,7 +1840,7 @@
  *
  * Requires: MBEDTLS_SSL_MAX_FRAGMENT_LENGTH
  */
-//#define MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH
+#define MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH

 /**
  * \def MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN
@@ -3162,7 +3162,7 @@
  *
  * This module adds support for SHA-512.
  */
-#define MBEDTLS_SHA512_C
+// #define MBEDTLS_SHA512_C

 /**
  * \def MBEDTLS_SHA512_USE_A64_CRYPTO_IF_PRESENT
@@ -3791,7 +3791,7 @@
  *
  * Uncomment to set the maximum plaintext size of the outgoing I/O buffer.
  */
-//#define MBEDTLS_SSL_OUT_CONTENT_LEN             16384
+#define MBEDTLS_SSL_OUT_CONTENT_LEN             4096

 /** \def MBEDTLS_SSL_DTLS_MAX_BUFFERING
  *