Fixed problem with multiple load-unload of comp zlib

Signed-off-by: NRich Salz <rsalz@akamai.com>
Reviewed-by: NTim Hudson <tjh@openssl.org>

crypto/comp/c_zlib.c

@@ -309,7 +309,9 @@ COMP_METHOD *COMP_zlib(void)
 void COMP_zlib_cleanup(void)
 {
 #ifdef ZLIB_SHARED
-    DSO_free(zlib_dso);
+    if (zlib_dso != NULL)
+        DSO_free(zlib_dso);
+    zlib_dso = NULL;
 #endif
 }