Fix segfault when interpreting bad debug_file

BUG=chromium:661037

Change-Id: Ia4da0bd9787c232a6a199cfdfccfbed60c2515c2
Reviewed-on: https://chromium-review.googlesource.com/450090
Reviewed-by: Joshua Peraza <jperaza@chromium.org>

src/processor/minidump.cc

@@ -1988,11 +1988,13 @@ string MinidumpModule::debug_file() const {
           // GetMiscRecord already byte-swapped the data[] field if it contains
           // UTF-16, so pass false as the swap argument.
           scoped_ptr<string> new_file(UTF16ToUTF8(string_utf16, false));
+          if (new_file.get() != nullptr) {
             file = *new_file;
           }
         }
       }
     }
+  }
 
   // Relatively common case
   BPLOG_IF(INFO, file.empty()) << "MinidumpModule could not determine "