fix invalid pointer comparison in attach

src/aHimawariFile.cxx

@@ -16,12 +16,12 @@ aHimawariFile *aHimawariFile::attach(const char *pathname, const struct Himawari
 
     aHimawariFile *ret=NULL;
 	ret = attachHCast(pathname, optional_sentinels);
-	if (ret>0)
+	if (ret != 0)
 		return ret;
 
 	// for now, just attempt til we succeed
 	ret = attachHSD(pathname, optional_sentinels);
-	if (ret>0)
+	if (ret != 0)
 		return ret;
 	// add other file types here