Check if num is 0 before trying to malloc memory. Otherwise for client hellos without extensions SSL_client_hello_get1_extensions_present will return MALLOC_FAILURE.

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8756)

doc/man3/SSL_CTX_set_client_hello_cb.pod

@@ -65,6 +65,8 @@ both required, and on success the caller must release the storage allocated for
 B<*out> using OPENSSL_free().  The contents of B<*out> is an array of integers
 holding the numerical value of the TLS extension types in the order they appear
 in the ClientHello.  B<*outlen> contains the number of elements in the array.
+In situations when the ClientHello has no extensions, the function will return
+success with B<*out> set to NULL and B<*outlen> set to 0.
 
 =head1 NOTES
 

ssl/ssl_lib.c

@@ -5140,6 +5140,11 @@ int SSL_client_hello_get1_extensions_present(SSL *s, int **out, size_t *outlen)
         if (ext->present)
             num++;
     }
+    if (num == 0) {
+        *out = NULL;
+        *outlen = 0;
+        return 1;
+    }
     if ((present = OPENSSL_malloc(sizeof(*present) * num)) == NULL) {
         SSLerr(SSL_F_SSL_CLIENT_HELLO_GET1_EXTENSIONS_PRESENT,
                ERR_R_MALLOC_FAILURE);