Avoid out-of-bounds access via std::vector::operator[].

This triggered C++ STL debug checks, and actually is undefined behaviour,
although it didn't really cause any problems so far.
Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/sfoutputstream.cc

@@ -110,7 +110,7 @@ SFOutputStream::write_frames (const vector<float>& samples)
     }
 
   sf_count_t frames = samples.size() / m_n_channels;
-  sf_count_t count = sf_writef_int (m_sndfile, &isamples[0], frames);
+  sf_count_t count = sf_writef_int (m_sndfile, isamples.data(), frames);
 
   if (sf_error (m_sndfile))
     return Error (sf_strerror (m_sndfile));

src/stdoutwavoutputstream.cc

@@ -136,7 +136,7 @@ StdoutWavOutputStream::write_frames (const vector<float>& samples)
 
   m_raw_converter->to_raw (samples, output_bytes);
 
-  fwrite (&output_bytes[0], 1, output_bytes.size(), stdout);
+  fwrite (output_bytes.data(), 1, output_bytes.size(), stdout);
   if (ferror (stdout))
     return Error ("write sample data failed");
 

src/wmadd.cc

@@ -214,9 +214,9 @@ public:
   {
     const size_t synth_frame_sz = Params::frame_size * n_channels;
     /* move frame 1 and frame 2 to frame 0 and frame 1 */
-    std::copy (&synth_samples[synth_frame_sz], &synth_samples[synth_frame_sz * 3], &synth_samples[0]);
+    std::copy (synth_samples.begin() + synth_frame_sz, synth_samples.end(), synth_samples.begin());
     /* zero out frame 2 */
-    std::fill (&synth_samples[synth_frame_sz * 2], &synth_samples[synth_frame_sz * 3], 0);
+    std::fill (synth_samples.begin() + synth_frame_sz * 2, synth_samples.end(), 0);
     for (int ch = 0; ch < n_channels; ch++)
       {
         /* mix watermark signal to output frame */
@@ -418,7 +418,7 @@ public:
       }
 
     uint start = 0;
-    do
+    while (start != frames.size() / n_channels)
       {
         const int out_count = Params::frame_size;
         float out[out_count * n_channels];
@@ -435,7 +435,6 @@ public:
 
         start = frames.size() / n_channels - m_resampler.inp_count;
       }
-    while (start != frames.size() / n_channels);
   }
   vector<float>
   read_frames (size_t frames)