Fix warnings Dereference of null pointer

include/rapidjson/internal/itoa.h

@@ -37,6 +37,8 @@ inline const char* GetDigitsLut() {
 }
 
 inline char* u32toa(uint32_t value, char* buffer) {
+    RAPIDJSON_ASSERT(buffer != 0);
+
     const char* cDigitsLut = GetDigitsLut();
 
     if (value < 10000) {
@@ -111,6 +113,7 @@ inline char* u32toa(uint32_t value, char* buffer) {
 }
 
 inline char* i32toa(int32_t value, char* buffer) {
+    RAPIDJSON_ASSERT(buffer != 0);
     uint32_t u = static_cast<uint32_t>(value);
     if (value < 0) {
         *buffer++ = '-';
@@ -121,6 +124,7 @@ inline char* i32toa(int32_t value, char* buffer) {
 }
 
 inline char* u64toa(uint64_t value, char* buffer) {
+    RAPIDJSON_ASSERT(buffer != 0);
     const char* cDigitsLut = GetDigitsLut();
     const uint64_t  kTen8 = 100000000;
     const uint64_t  kTen9 = kTen8 * 10;
@@ -289,6 +293,7 @@ inline char* u64toa(uint64_t value, char* buffer) {
 }
 
 inline char* i64toa(int64_t value, char* buffer) {
+    RAPIDJSON_ASSERT(buffer != 0);
     uint64_t u = static_cast<uint64_t>(value);
     if (value < 0) {
         *buffer++ = '-';