Try to fix a potential set fault on some compiler

Merge the fix from https://github.com/miloyip/itoa-benchmark/issues/8

include/rapidjson/internal/itoa.h

@@ -109,12 +109,13 @@ inline char* u32toa(uint32_t value, char* buffer) {
 }
 
 inline char* i32toa(int32_t value, char* buffer) {
+    uint32_t u = static_cast<uint32_t>(value);
     if (value < 0) {
         *buffer++ = '-';
-        value = -value;
+        u = ~u + 1;
     }
 
-    return u32toa(static_cast<uint32_t>(value), buffer);
+    return u32toa(u, buffer);
 }
 
 inline char* u64toa(uint64_t value, char* buffer) {
@@ -286,12 +287,13 @@ inline char* u64toa(uint64_t value, char* buffer) {
 }
 
 inline char* i64toa(int64_t value, char* buffer) {
+    uint64_t u = static_cast<uint64_t>(value);
     if (value < 0) {
         *buffer++ = '-';
-        value = -value;
+        u = ~u + 1;
     }
 
-    return u64toa(static_cast<uint64_t>(value), buffer);
+    return u64toa(u, buffer);
 }
 
 } // namespace internal