bootchart: use 'n/a' if PRETTY_NAME is not found

Spotted with coverity. If parsing both /etc/os-release and
/usr/lib/os-release fails then null would be passed on. The calls
to parse the two files are allowed to fail. A empty /etc may not
have had the /etc/os-release symlink restored yet and we just
try again in the loop. If for whatever reason that does not happen
then we now pass on 'n/a' instead of null.

src/bootchart/bootchart.c

@@ -471,7 +471,7 @@ int main(int argc, char *argv[]) {
                 exit (EXIT_FAILURE);
         }
 
-        svg_do(build);
+        svg_do(strna(build));
 
         fprintf(stderr, "systemd-bootchart wrote %s\n", output_file);