Compiler warning fix.

Assigning the const value zero to a pointer to function
results in a null pointer value assignment to the function
pointer.

Assignment of any nonzero value is what should result in a
implementation compiler dependent result.

Since what we want to do here is the first case, this should
not trigger compiler warnings related with conversions from
'pointer to data' to 'pointer to function'.

Our autobuild test suite will judge.

lib/ldap.c

@@ -189,7 +189,7 @@ static void DynaClose(void)
 
 static dynafunc DynaGetFunction(const char *name)
 {
-  dynafunc func = (void *(*)(void *))NULL;
+  dynafunc func = (void *(*)(void *))0;
 
 #if defined(HAVE_DLOPEN) || defined(HAVE_LIBDL)
   if (libldap) {