better error control in fimg_destroy

lib/fimg-core.c

@@ -106,7 +106,6 @@ return 0;
 }
 
 /* --------------------------------------------------------------------- */
-
 int fimg_destroy(FloatImg *fimg)
 {
 
@@ -114,10 +113,18 @@ int fimg_destroy(FloatImg *fimg)
 fprintf(stderr, ">>> %-25s ( %p )\n", __func__, fimg);
 #endif
 
+if (NULL == fimg) {
+	fprintf(stderr, "%s : parameter is null\n", __func__);
+	return -1;
+	}
+
 if ( ! fimg_type_is_valid(fimg->type) ) {
 	return -2;
 	}
-
+if (NULL == fimg->R) {
+	fprintf(stderr, "%s : %p already freeed\n", __func__, fimg);
+	return -3;
+	}
 free(fimg->R);
 memset(fimg, 0, sizeof(FloatImg));