bugreport.c: fix a crash in git bugreport with --no-suffix option

executing `git bugreport --no-suffix` led to a segmentation fault
due to strbuf_addftime() being called with a NULL option_suffix
variable. This occurs because negating the "--[no-]suffix" option
causes the parser to set option_suffix to NULL, which is not
handled prior to calling strbuf_addftime().

Signed-off-by: Jiamu Sun <barroit@linux.com>

builtin/bugreport.c

@@ -138,8 +138,11 @@ int cmd_bugreport(int argc, const char **argv, const char *prefix)
 	strbuf_complete(&report_path, '/');
 	output_path_len = report_path.len;
 
-	strbuf_addstr(&report_path, "git-bugreport-");
-	strbuf_addftime(&report_path, option_suffix, localtime_r(&now, &tm), 0, 0);
+	strbuf_addstr(&report_path, "git-bugreport");
+	if (option_suffix) {
+		strbuf_addch(&report_path, '-');
+		strbuf_addftime(&report_path, option_suffix, localtime_r(&now, &tm), 0, 0);
+	}
 	strbuf_addstr(&report_path, ".txt");
 
 	switch (safe_create_leading_directories(report_path.buf)) {