Fix unsiged underflow

src/google/protobuf/stubs/io_win32_unittest.cc

diff --git a/src/google/protobuf/stubs/io_win32_unittest.cc b/src/google/protobuf/stubs/io_win32_unittest.cc
index 4777ee7..3ef6c75 100644
--- a/src/google/protobuf/stubs/io_win32_unittest.cc
+++ b/src/google/protobuf/stubs/io_win32_unittest.cc
@@ -85,7 +85,7 @@
 
 namespace {
 void StripTrailingSlashes(string* str) {
-  size_t i = str->size() - 1;
+  int i = str->size() - 1;
   for (; i >= 0 && ((*str)[i] == '/' || (*str)[i] == '\\'); --i) {}
   str->resize(i+1);
 }