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Fix collapse_masks overflow for mono.

We were always storing collapse_masks[i*C+1], which could have
 overflowed the buffer on the last band in mono.
This also moves the stores outside the conditional, which allows
 common code to be used for masking the high bits, address
 generation, etc.

--- a/libcelt/bands.c

+++ b/libcelt/bands.c

@@ -1209,14 +1209,14 @@

          y_cm = quant_band(encode, m, i, Y, NULL, N, b/2, spread, B, intensity, tf_change,

                effective_lowband != -1 ? norm2+effective_lowband : NULL, resynth, ec, &remaining_bits, LM,

                norm2+M*eBands[i], bandE, 0, seed, Q15ONE, lowband_scratch, y_cm);

-         collapse_masks[i*2+0] = (unsigned char)(x_cm&(1<<B)-1);

-         collapse_masks[i*2+1] = (unsigned char)(y_cm&(1<<B)-1);

       } else {

          x_cm = quant_band(encode, m, i, X, Y, N, b, spread, B, intensity, tf_change,

                effective_lowband != -1 ? norm+effective_lowband : NULL, resynth, ec, &remaining_bits, LM,

                norm+M*eBands[i], bandE, 0, seed, Q15ONE, lowband_scratch, x_cm|y_cm);

-         collapse_masks[i*C+1] = collapse_masks[i*C+0] = (unsigned char)(x_cm&(1<<B)-1);

+         y_cm = x_cm;

       }

+      collapse_masks[i*C+0] = (unsigned char)(x_cm&(1<<B)-1);

+      collapse_masks[i*C+C-1] = (unsigned char)(y_cm&(1<<B)-1);

       balance += pulses[i] + tell;

       /* Update the folding position only as long as we have 1 bit/sample depth */