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Increase buffer size for SINC converters

To fulfil the condition `samples_in_hand <= half_filter_chan_len` after
the first call to prepare_data the buffer size `b_len` must be bigger
than 3*half_filter_chan_len.
Tracing that back results in the implemented calculation.
Fixes #92

--- a/src/src_sinc.c

+++ b/src/src_sinc.c

@@ -212,9 +212,10 @@

 	** a better way. Need to look at prepare_data () at the same time.

 	*/

-	temp_filter.b_len = (int) lrint (2.5 * temp_filter.coeff_half_len / (temp_filter.index_inc * 1.0) * SRC_MAX_RATIO) ;

+	temp_filter.b_len = 3 * (int) lrint ((temp_filter.coeff_half_len + 2.0) / temp_filter.index_inc * SRC_MAX_RATIO + 1) ;

 	temp_filter.b_len = MAX (temp_filter.b_len, 4096) ;

 	temp_filter.b_len *= temp_filter.channels ;

+	temp_filter.b_len += 1 ; // There is a <= check against samples_in_hand requiring a buffer bigger than the calculation above

 	if ((filter = ZERO_ALLOC (SINC_FILTER, sizeof (SINC_FILTER) + sizeof (filter->buffer [0]) * (temp_filter.b_len + temp_filter.channels))) == NULL)

 		return SRC_ERR_MALLOC_FAILED ;