Ensure bandwidth is positive. Use engineering units for bandwidths.

sdrgui/gui/glspectrumview.cpp

@@ -2307,8 +2307,9 @@ void GLSpectrumView::measure3dBBandwidth()
     }
 
     // Calcualte bandwidth
-    float hzPerBin = m_sampleRate / (float) m_fftSize;
     int bins = rightBin - leftBin - 1;
+    bins = std::max(1, bins);
+    float hzPerBin = m_sampleRate / (float) m_fftSize;
     float bandwidth = bins * hzPerBin;
     int centerBin = leftBin + (rightBin - leftBin) / 2;
     float centerFrequency = binToFrequency(centerBin);

sdrgui/gui/spectrummeasurements.cpp

@@ -350,7 +350,7 @@ void SpectrumMeasurements::createAdjacentChannelPowerTable()
 void SpectrumMeasurements::createOccupiedBandwidthTable()
 {
     QStringList rows = {"Occupied B/W"};
-    QStringList units = {" Hz"};
+    QStringList units = {"Hz"};
 
     createMeasurementsTable(rows, units);
 }
@@ -358,7 +358,7 @@ void SpectrumMeasurements::createOccupiedBandwidthTable()
 void SpectrumMeasurements::create3dBBandwidthTable()
 {
     QStringList rows = {"3dB B/W"};
-    QStringList units = {" Hz"};
+    QStringList units = {"Hz"};
 
     createMeasurementsTable(rows, units);
 }

sdrgui/gui/spectrummeasurementsdialog.ui

@@ -157,7 +157,7 @@
        </item>
        <item>
         <property name="text">
-         <string>Occupied b/w</string>
+         <string>Occupied bandwidth</string>
         </property>
        </item>
        <item>