Run init/free to the backup context for GetDigest. (#19233)

CYW30739 platform specific SHA256 implementations need
mbedtls_sha256_init/mbedtls_sha256_free functions called to do
initialization and cleanup.

src/crypto/CHIPCryptoPALmbedTLS.cpp

@@ -290,13 +290,15 @@ CHIP_ERROR Hash_SHA256_stream::GetDigest(MutableByteSpan & out_buffer)
 
     // Back-up context as we are about to finalize the hash to extract digest.
     mbedtls_sha256_context previous_ctx;
+    mbedtls_sha256_init(&previous_ctx);
     mbedtls_sha256_clone(&previous_ctx, context);
 
     // Pad + compute digest, then finalize context. It is restored next line to continue.
     CHIP_ERROR result = Finish(out_buffer);
 
     // Restore context prior to finalization.
     mbedtls_sha256_clone(context, &previous_ctx);
+    mbedtls_sha256_free(&previous_ctx);
 
     return result;
 }