test: Fix shared random buffer overrun

Encoded commit has an extra byte at the end for the NUL terminated byte and
the test was overrunning the payload buffer by one byte.

Found by Coverity issue 1362984.

Fixes #19567

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/test/test_shared_random.c

@@ -932,13 +932,14 @@ test_utils(void *arg)
   /* Testing commitments_are_the_same(). Currently, the check is to test the
    * value of the encoded commit so let's make sure that actually works. */
   {
-    /* Payload of 55 bytes that is the length of
-     * sr_commit_t->encoded_commit. */
+    /* Payload of 57 bytes that is the length of sr_commit_t->encoded_commit.
+     * 56 bytes of payload and a NUL terminated byte at the end ('\x00')
+     * which comes down to SR_COMMIT_BASE64_LEN + 1. */
     const char *payload =
       "\x5d\xb9\x60\xb6\xcc\x51\x68\x52\x31\xd9\x88\x88\x71\x71\xe0\x30"
       "\x59\x55\x7f\xcd\x61\xc0\x4b\x05\xb8\xcd\xc1\x48\xe9\xcd\x16\x1f"
       "\x70\x15\x0c\xfc\xd3\x1a\x75\xd0\x93\x6c\xc4\xe0\x5c\xbe\xe2\x18"
-      "\xc7\xaf\x72\xb6\x7c\x9b\x52";
+      "\xc7\xaf\x72\xb6\x7c\x9b\x52\x00";
     sr_commit_t commit1, commit2;
     memcpy(commit1.encoded_commit, payload, sizeof(commit1.encoded_commit));
     memcpy(commit2.encoded_commit, payload, sizeof(commit2.encoded_commit));