Use secp256k1_memclear() to clear stack memory instead of memset()

All of the invocations of secp256k1_memclear() operate on stack
memory and happen after the function is done with the memory object.
This commit replaces existing memset() invocations and also adds
secp256k1_memclear() to code locations where clearing was missing;
there is no guarantee that this commit covers all code locations
where clearing is necessary.

Co-Authored-By: isle2983 <isle2983@yahoo.com>

src/modules/ellswift/main_impl.h

@@ -580,7 +580,7 @@ int secp256k1_ellswift_xdh(const secp256k1_context *ctx, unsigned char *output,
     /* Invoke hasher */
     ret = hashfp(output, sx, ell_a64, ell_b64, data);
 
-    memset(sx, 0, 32);
+    secp256k1_memclear(sx, sizeof(sx));
     secp256k1_fe_clear(&px);
     secp256k1_scalar_clear(&s);