MINOR: ist: Add `int isttest(const struct ist)`

`isttest` returns whether the `.ptr` is non-null.

include/common/ist.h

@@ -186,6 +186,12 @@ static inline struct ist ist2(const void *ptr, size_t len)
 	return (struct ist){ .ptr = (char *)ptr, .len = len };
 }
 
+/* returns the result of `ist.ptr != NULL` */
+static inline int isttest(const struct ist ist)
+{
+	return ist.ptr != NULL;
+}
+
 /* This function MODIFIES the string to add a zero AFTER the end, and returns
  * the start pointer. The purpose is to use it on strings extracted by parsers
  * from larger strings cut with delimiters that are not important and can be

src/http_ana.c

@@ -2787,7 +2787,7 @@ int http_res_set_status(unsigned int status, struct ist reason, struct stream *s
 	trash.data = res - trash.area;
 
 	/* Do we have a custom reason format string? */
-	if (reason.ptr == NULL) {
+	if (!isttest(reason)) {
 		const char *str = http_get_reason(status);
 		reason = ist2(str, strlen(str));
 	}