Use safer sizeof variant in malloc

For a local variable:
        TYPE *p;
Allocations like this are "risky":
        p = OPENSSL_malloc(sizeof(TYPE));
if the type of p changes, and the malloc call isn't updated, you
could get memory corruption.  Instead do this:
        p = OPENSSL_malloc(sizeof(*p));
Also fixed a few memset() calls that I noticed while doing this.

Reviewed-by: Richard Levitte <levitte@openssl.org>

test/ectest.c

@@ -1359,9 +1359,7 @@ static void internal_curve_test(void)
     int ok = 1;
 
     crv_len = EC_get_builtin_curves(NULL, 0);
-
-    curves = OPENSSL_malloc(sizeof(EC_builtin_curve) * crv_len);
-
+    curves = OPENSSL_malloc(sizeof(*curves) * crv_len);
     if (curves == NULL)
         return;