[7.1] 0fb3baff7 Clean up assertions in http_hdr_flags()

[Martin Blix Grydeland]

commit 0fb3baff7963604a55be0ed6ebdf1e4654ead219
Author: Martin Blix Grydeland <martin at varnish-software.com>
Date:   Thu Aug 4 11:04:37 2022 +0200

    Clean up assertions in http_hdr_flags()
    
    The input argument assertions and checks in http_hdr_flags() were
    misleading and lacking. With this patch it returns (NULL) on either input
    being NULL, and also when called with an empty string instead of
    asserting.

diff --git a/bin/varnishd/cache/cache_http.c b/bin/varnishd/cache/cache_http.c
index 3f5ee0384..194055c3c 100644
--- a/bin/varnishd/cache/cache_http.c
+++ b/bin/varnishd/cache/cache_http.c
@@ -140,9 +140,9 @@ http_hdr_flags(const char *b, const char *e)
 	unsigned u;
 	struct http_hdrflg *retval;
 
-	if (e == NULL)
-		return(NULL);
-	assert(e > b);
+	if (b == NULL || e == NULL)
+		return (NULL);
+	assert(b <= e);
 	u = (unsigned)(e - b);
 	assert(b + u == e);
 	if (u < 2 || u > 19)		// MIN_WORD_LENGTH & MAX_WORD_LENGTH