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>

crypto/LPdir_vms.c

@@ -104,12 +104,12 @@ const char *LP_find_file(LP_DIR_CTX **ctx, const char *directory)
             return 0;
         }
 
-        *ctx = (LP_DIR_CTX *)malloc(sizeof(LP_DIR_CTX));
+        *ctx = malloc(sizeof(**ctx));
         if (*ctx == NULL) {
             errno = ENOMEM;
             return 0;
         }
-        memset(*ctx, '\0', sizeof(LP_DIR_CTX));
+        memset(*ctx, '\0', sizeof(**ctx));
 
         strcpy((*ctx)->filespec, directory);
         strcat((*ctx)->filespec, "*.*;");