BUILD/MINOR: systemd: fix compiler warning about unused result

There is a compiler warning after commit 1b6e75fa84 ("MEDIUM: haproxy-
systemd-wrapper: Use haproxy in same directory"):

src/haproxy-systemd-wrapper.c: In function ‘locate_haproxy’:
src/haproxy-systemd-wrapper.c:28:10: warning: ignoring return value of ‘readlink’, declared with attribute warn_unused_result [-Wunused-result]

Fix the compiler warning by checking the return value of readlink().
diff --git a/src/haproxy-systemd-wrapper.c b/src/haproxy-systemd-wrapper.c
index 4ca86dd..c63f41f 100644
--- a/src/haproxy-systemd-wrapper.c
+++ b/src/haproxy-systemd-wrapper.c
@@ -24,9 +24,9 @@
 
 static void locate_haproxy(char *buffer, size_t buffer_size)
 {
-	char* end;
-	readlink("/proc/self/exe", buffer, buffer_size);
-	end = strrchr(buffer, '/');
+	char* end = NULL;
+	if (readlink("/proc/self/exe", buffer, buffer_size) > 0)
+		end = strrchr(buffer, '/');
 	if (end == NULL)
 		strncpy(buffer, "/usr/sbin/haproxy", buffer_size);
 	end[1] = '\0';