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/ecdh/main_impl.h

@@ -61,9 +61,10 @@ int secp256k1_ecdh(const secp256k1_context* ctx, unsigned char *output, const se
 
     ret = hashfp(output, x, y, data);
 
-    memset(x, 0, 32);
-    memset(y, 0, 32);
+    secp256k1_memclear(x, sizeof(x));
+    secp256k1_memclear(y, sizeof(y));
     secp256k1_scalar_clear(&s);
+    secp256k1_ge_clear(&pt);
 
     return !!ret & !overflow;
 }