Add and use OPENSSL_zalloc

There are many places (nearly 50) where we malloc and then memset.
Add an OPENSSL_zalloc routine to encapsulate that.
(Missed one conversion; thanks Richard)
Also fixes GH328

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

crypto/comp/c_zlib.c

@@ -91,9 +91,7 @@ static void *zlib_zalloc(void *opaque, unsigned int no, unsigned int size)
 {
     void *p;
 
-    p = OPENSSL_malloc(no * size);
-    if (p)
-        memset(p, 0, no * size);
+    p = OPENSSL_zalloc(no * size);
     return p;
 }