remove duplicated allocations

Summary:
report_number is zero before the conditions
Closes https://github.com/facebook/infer/pull/74
Github Author: Chase choi <cs09gi@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

infer/bin/inferTraceBugs

@@ -187,13 +187,10 @@ class Selector(object):
         if len(self) > 1:
             report_number_str = raw_input(
                 'Choose report to display (default=0): ')
-            if report_number_str == '':
+            if report_number_str != '':
-                report_number = 0
-            else:
                 report_number = self.parse_report_number(report_number_str)
         elif len(self) == 1:
             print('Auto-selecting the only report.')
-            report_number = 0
 
         return report_number