modais: Initialize AIS modulator sample variable

Coverity reported an uninitialized value use in AISModSource::modulateSample().
The Gaussian filter output variable was only assigned during transmission,
but was used afterwards when the modulator was idle or waiting.

Initialize the variable to zero to match the idle sample behavior.

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

plugins/channeltx/modais/aismodsource.cpp

@@ -154,7 +154,7 @@ void AISModSource::sampleToScope(Complex sample)
 
 void AISModSource::modulateSample()
 {
-    Real mod;
+    Real mod = 0.0f;
     Real linearRampGain;
 
     if ((m_state == idle) || (m_state == wait))