fix: security issues

src/file.c

@@ -61,7 +61,7 @@ bool file_update(File *file) {
   // close file
   fclose(file_pointer);
   // append null byte
-  file->content[length] = '\0';
+  file->content[length] = 0;
   // read last update time
   file->last_write = get_file_time(file);
 

src/tempo.c

@@ -52,6 +52,10 @@ static long get_average_tap_duration(Tempo tempo) {
     running_total += tempo.tap_durations[i];
   }
 
+  if (amount == 0) {
+    amount = 1; // should never happen
+  }
+
   average_tap_duration = running_total / amount;
   if (average_tap_duration < MIN_BEAT_LENGTH) {
     return MIN_BEAT_LENGTH;