sfeed_curses: pedantic fix for UB with an empty URL file

When a new URL file is used with no URL entries then NULL is passed to qsort()
and bsearch(). This is reported by clang UBsan as undefined behaviour
(debatable), but no issue in practise with many implementations. Fix it anyway.

To reproduce with clang UBsan:
Compile with clang or gcc with CFLAGS and LDFLAGS -fsanitize=undefined
touch /tmp/urls # new file which should be empty.
SFEED_URL_FILE=/tmp/urls sfeed_curses 2>/tmp/log
^D
q
cat /tmp/log

1 files changed, 4 insertions, 2 deletions

diff --git a/sfeed_curses.c b/sfeed_curses.c
index d4ca13d..6adbfec 100644
--- a/sfeed_curses.c
+++ b/sfeed_curses.c
@@ -1886,7 +1886,8 @@ urls_cmp(const void *v1, const void *v2)
 int
 urls_isnew(const char *url)
 {
-	return bsearch(&url, urls, nurls, sizeof(char *), urls_cmp) == NULL;
+	return (!nurls ||
+	       bsearch(&url, urls, nurls, sizeof(char *), urls_cmp) == NULL);
 }
 
 void
@@ -1928,7 +1929,8 @@ urls_read(void)
 	fclose(fp);
 	free(line);
 
-	qsort(urls, nurls, sizeof(char *), urls_cmp);
+	if (nurls > 0)
+		qsort(urls, nurls, sizeof(char *), urls_cmp);
 }
 
 int