breakpad: fix unittest failure when building with clang.

In C/C++, the result of signed integer overflow is undefined.

The expression "base + size - 1" is parsed as "(base + size) - 1", and
"base + size" can overflow even if "base + (size - 1)" <= INT_MAX.

See http://g/c-compiler-chrome/461JohPKakE/JI3rEBg6FwAJ for more.

BUG=None
TEST='CC=clang CXX=clang++ ./configure && make check'
R=vapier@chromium.org

Review URL: https://codereview.chromium.org/1591793002 .

src/processor/range_map-inl.h

@@ -50,7 +50,7 @@ template<typename AddressType, typename EntryType>
 bool RangeMap<AddressType, EntryType>::StoreRange(const AddressType &base,
                                                   const AddressType &size,
                                                   const EntryType &entry) {
-  AddressType high = base + size - 1;
+  AddressType high = base + (size - 1);
 
   // Check for undersize or overflow.
   if (size <= 0 || high < base) {