[inferTraceBugs] fix selected/displayed bug mismatch

Summary:
inferTraceBugs filters the bugs to only show those which have a trace attached
to them. However, the number selected by the user would then be used in the
unfiltered list of bugs. This led to sometimes selecting the wrong bug from the
text interface and from the --select option.

infer/bin/inferTraceBugs

@@ -177,22 +177,22 @@ class Selector(object):
                   describe_report(report, n_length + 2))
             n += 1
 
-    def prompt_number(self):
+    def prompt_report(self):
+        report_number = 0
         if self.args.select is not None:
-            return self.parse_report_number(self.args.select[0], True)
-
-        self.show_choices()
+            report_number = self.parse_report_number(self.args.select[0], True)
+        else:
+            self.show_choices()
 
-        report_number = 0
-        if len(self) > 1:
-            report_number_str = raw_input(
-                'Choose report to display (default=0): ')
-            if report_number_str != '':
-                report_number = self.parse_report_number(report_number_str)
-        elif len(self) == 1:
-            print('Auto-selecting the only report.')
+            if len(self) > 1:
+                report_number_str = raw_input(
+                    'Choose report to display (default=0): ')
+                if report_number_str != '':
+                    report_number = self.parse_report_number(report_number_str)
+            elif len(self) == 1:
+                print('Auto-selecting the only report.')
 
-        return report_number
+        return self.reports[report_number]
 
     def prompt_level(self):
         if self.args.max_level is not None:
@@ -258,11 +258,11 @@ def main():
         sel.show_choices()
         exit(0)
 
-    report_number = sel.prompt_number()
+    report = sel.prompt_report()
     max_level = sel.prompt_level()
 
     tracer = Tracer(args, max_level)
-    tracer.build_report(reports[report_number])
+    tracer.build_report(report)
     print(tracer)