sr_analog_to_float(): Fix a compiler warning (-Wshadow).

  src/analog.c: In function ‘sr_analog_to_float’:
  src/analog.c:194:9: warning: declaration of ‘offset’ shadows a previous local [-Wshadow]
   float offset = analog->encoding->offset.p / (float)analog->encoding->offset.q;
         ^~~~~~
  src/analog.c:177:8: note: shadowed declaration is here
  float offset;
        ^~~~~~