builtin/bugreport: don't leak prefixed filename

prefix_filename() returns newly allocated memory, and strbuf_addstr()
doesn't take ownership of its inputs. Therefore we have to make sure to
store and free prefix_filename()'s result.

As this leak is in cmd_bugreport(), we could just as well UNLEAK the
prefix - but there's no good reason not to just free it properly. This
leak was found while running t0091, see output below:

Direct leak of 24 byte(s) in 1 object(s) allocated from:
    #0 0x49ab79 in realloc /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:164:3
    #1 0x9acc66 in xrealloc wrapper.c:126:8
    #2 0x93baed in strbuf_grow strbuf.c:98:2
    #3 0x93c6ea in strbuf_add strbuf.c:295:2
    #4 0x69f162 in strbuf_addstr ./strbuf.h:304:2
    #5 0x69f083 in prefix_filename abspath.c:277:2
    #6 0x4fb275 in cmd_bugreport builtin/bugreport.c:146:9
    #7 0x4cd91d in run_builtin git.c:467:11
    #8 0x4cb5f3 in handle_builtin git.c:719:3
    #9 0x4ccf47 in run_argv git.c:808:4
    #10 0x4caf49 in cmd_main git.c:939:19
    #11 0x69df9e in main common-main.c:52:11
    #12 0x7f523a987349 in __libc_start_main (/lib64/libc.so.6+0x24349)

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/bugreport.c

@@ -129,6 +129,7 @@ int cmd_bugreport(int argc, const char **argv, const char *prefix)
 	char *option_output = NULL;
 	char *option_suffix = "%Y-%m-%d-%H%M";
 	const char *user_relative_path = NULL;
+	char *prefixed_filename;
 
 	const struct option bugreport_options[] = {
 		OPT_STRING('o', "output-directory", &option_output, N_("path"),
@@ -142,9 +143,9 @@ int cmd_bugreport(int argc, const char **argv, const char *prefix)
 			     bugreport_usage, 0);
 
 	/* Prepare the path to put the result */
-	strbuf_addstr(&report_path,
-		      prefix_filename(prefix,
-				      option_output ? option_output : ""));
+	prefixed_filename = prefix_filename(prefix,
+					    option_output ? option_output : "");
+	strbuf_addstr(&report_path, prefixed_filename);
 	strbuf_complete(&report_path, '/');
 
 	strbuf_addstr(&report_path, "git-bugreport-");
@@ -189,6 +190,7 @@ int cmd_bugreport(int argc, const char **argv, const char *prefix)
 	fprintf(stderr, _("Created new report at '%s'.\n"),
 		user_relative_path);
 
+	free(prefixed_filename);
 	UNLEAK(buffer);
 	UNLEAK(report_path);
 	return !!launch_editor(report_path.buf, NULL, NULL);