leansdr: Properly initialize QPSK hist buffer

Replace raw memset() initialization of hist with C++ value initialization.
This avoids bypassing std::complex object initialization and ensures the
history buffer contains valid constructed objects when HIST_FLOAT is enabled.

The previous memset() relied on the in-memory representation of
std::complex<float> and could leave non-trivial objects improperly
initialized. While this typically behaved as expected with common
implementations, it was not valid C++ object initialization.

pointed out by cppcheck as:
    Using 'memset' on struct that contains a 'std::complex'

Signed-off-by: Robin Getz <rgetz503@gmail.com>

plugins/channelrx/demoddatv/leansdr/sdr.h

@@ -1472,6 +1472,7 @@ struct fast_qpsk_receiver : runnable
         meas_decimation(1048576),
         pll_adjustment(1.0),
         allow_drift(false),
+        hist{},
         in(_in),
         out(_out, chunk_size),
         mu(0),
@@ -1482,7 +1483,6 @@ struct fast_qpsk_receiver : runnable
         set_freq(0);
         freq_out = _freq_out ? new pipewriter<float>(*_freq_out) : nullptr;
         cstln_out = _cstln_out ? new pipewriter<std::complex<T>>(*_cstln_out) : nullptr;
-        memset(hist, 0, sizeof(hist));
         init_lookup_tables();
     }