apps: catch corner cases for file= mode in osmocom_fft

verified for gqrx and sdrangerlove as well:

file=/tmp/lte.cfile,rate=40e6,freq=1815e6,repeat=true,throttle=true

apps/osmocom_fft

@@ -113,7 +113,11 @@ class app_top_block(stdgui2.std_top_block, pubsub):
         if options.gain is None:
             # if no gain was specified, use the mid-point in dB
             r = self.src.get_gain_range()
-            options.gain = float(r.start()+r.stop())/2
+            try: # empty gain range returned in file= mode
+                options.gain = float(r.start()+r.stop())/2
+            except RuntimeError:
+                options.gain = 0
+                pass
 
         if options.center_freq is None:
             # if no freq was specified, use the mid-point in Hz
@@ -241,16 +245,21 @@ class app_top_block(stdgui2.std_top_block, pubsub):
         )
         freq_hbox.AddSpacer(5)
 
-        forms.slider(
-            parent=self.panel, sizer=freq_hbox,
-            proportion=3,
-            ps=self,
-            key=CENTER_FREQ_KEY,
-            minimum=self[FREQ_RANGE_KEY].start(),
-            maximum=self[FREQ_RANGE_KEY].stop(),
-            num_steps=101,
-        )
-        freq_hbox.AddSpacer(3)
+        try: # range.start() == range.stop() in file= mode
+
+            forms.slider(
+                parent=self.panel, sizer=freq_hbox,
+                proportion=3,
+                ps=self,
+                key=CENTER_FREQ_KEY,
+                minimum=self[FREQ_RANGE_KEY].start(),
+                maximum=self[FREQ_RANGE_KEY].stop(),
+                num_steps=101,
+            )
+            freq_hbox.AddSpacer(3)
+
+        except AssertionError:
+            pass
 
         corr_hbox.AddSpacer(3)
         forms.text_box(