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

`git bugreport` does not complain when `--no-suffix` is given, but
it leads to a segmentation fault as the it is not prepared to see a
NULL assigned to the option_suffix variable.

Signed-off-by: Jiamu Sun <barroit@linux.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bugreport.txt

@@ -8,7 +8,8 @@ git-bugreport - Collect information for user to file a bug report
 SYNOPSIS
 --------
 [verse]
-'git bugreport' [(-o | --output-directory) <path>] [(-s | --suffix) <format>]
+'git bugreport' [(-o | --output-directory) <path>]
+		[(-s | --suffix) <format> | --no-suffix]
 		[--diagnose[=<mode>]]
 
 DESCRIPTION
@@ -51,9 +52,12 @@ OPTIONS
 
 -s <format>::
 --suffix <format>::
+--no-suffix::
 	Specify an alternate suffix for the bugreport name, to create a file
 	named 'git-bugreport-<formatted-suffix>'. This should take the form of a
 	strftime(3) format string; the current local time will be used.
+	`--no-suffix` disables the suffix and the file is just named
+	`git-bugreport` without any disambiguation measure.
 
 --no-diagnose::
 --diagnose[=<mode>]::

builtin/bugreport.c

@@ -64,7 +64,8 @@ static void get_populated_hooks(struct strbuf *hook_info, int nongit)
 }
 
 static const char * const bugreport_usage[] = {
-	N_("git bugreport [(-o | --output-directory) <path>] [(-s | --suffix) <format>]\n"
+	N_("git bugreport [(-o | --output-directory) <path>]\n"
+	   "              [(-s | --suffix) <format> | --no-suffix]\n"
 	   "              [--diagnose[=<mode>]]"),
 	NULL
 };
@@ -138,8 +139,11 @@ 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_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)) {