gen_values: initialize local variable 'sample'

although in practice, this shouldn't happen -- I tried lowering the
sample rate, and even at 10 samples/sec with Martin M1, it worked, and
SSTV requires at least 4000 samples/sec

(1 sample/sec raises an error though, if you'd like to try)

pysstv/sstv.py

@@ -76,6 +76,7 @@ class SSTV(object):
         offset = 0
         samples = 0
         factor = 2 * pi / self.samples_per_sec
+        sample = 0
         for freq, msec in self.gen_freq_bits():
             samples += spms * msec
             tx = int(samples)