Fix thinko in new ECPG "PREPARE AS" code.

ecpg_register_prepared_stmt() is pretty obviously checking the wrong
variable while trying to detect malloc failure.  Error in commit
a1dc6ab46, spotted by Coverity.

src/interfaces/ecpg/ecpglib/prepare.c

@@ -85,7 +85,7 @@ ecpg_register_prepared_stmt(struct statement *stmt)
 		return false;
 
 	prep_stmt = (struct statement *) ecpg_alloc(sizeof(struct statement), lineno);
-	if (!stmt)
+	if (!prep_stmt)
 	{
 		ecpg_free(this);
 		return false;