Fix compilation with GCC 5.2

src/bignum.cc

Index: src/bignum.cc
diff --git a/src/bignum.cc b/src/bignum.cc
index 9baf77e7f28d64da14b660dfc47bd9c478c5f589..e7c6747665f12f1e5c6832cf1f54eea271f35d21 100644
--- a/src/bignum.cc
+++ b/src/bignum.cc
@@ -68,7 +68,9 @@ static uint64_t ReadUInt64(Vector<const char> buffer,
                            int from,
                            int digits_to_read) {
   uint64_t result = 0;
-  for (int i = from; i < from + digits_to_read; ++i) {
+  int to = from + digits_to_read;
+
+  for (int i = from; i < to; ++i) {
     int digit = buffer[i] - '0';
     DCHECK(0 <= digit && digit <= 9);
     result = result * 10 + digit;