[noup] zephyr: crypto: fix coverity issue in crypto_ecdh_set_peerkey

Fix INTEGER_OVERFLOW coverity issue.
crypto_ecdh_set_peerkey:
The decrement operator on the unsigned variable len
might result in an underflow.
Check value of the unsigned variable len before decrement.

Signed-off-by: Qingling Wu <qingling.wu@nxp.com>

Author: Qingling-Wu

src/crypto/crypto_mbedtls_alt.c

@@ -1927,6 +1927,8 @@ struct wpabuf *crypto_ecdh_set_peerkey(struct crypto_ecdh *ecdh, int inc_y, cons
         }
         else if (key[0] == 0x02 || key[0] == 0x03)
         {          /* (inc_y == 0) */
+            if(len == 0)
+                return NULL;
             --len; /*(repurpose len to prime_len)*/
 
             /* mbedtls_ecp_point_read_binary() does not currently support