corruption_test.cc: pass parameter by reference

Fix for:

[db/corruption_test.cc:134]: (performance) Function parameter
 'fname' should be passed by reference.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

db/corruption_test.cc

@@ -131,7 +131,7 @@ class CorruptionTest {
     ASSERT_GE(max_expected, correct);
   }
 
-  void CorruptFile(const std::string fname, int offset, int bytes_to_corrupt) {
+  void CorruptFile(const std::string& fname, int offset, int bytes_to_corrupt) {
     struct stat sbuf;
     if (stat(fname.c_str(), &sbuf) != 0) {
       const char* msg = strerror(errno);