Fix -Wsign-conversion warnings/errors

GCC 8 (incorrectly) warns about sign conversions in (constant)
array size expressions:

error: conversion to 'long unsigned int' from 'int' may
change the sign of the result [-Werror=sign-conversion]
     char schemaBuffer_[128 * 1024];

Make these expressions unsigned by adding a 'u' suffix to
the first operands.

include/rapidjson/schema.h

@@ -464,7 +464,7 @@ public:
                 enum_ = static_cast<uint64_t*>(allocator_->Malloc(sizeof(uint64_t) * v->Size()));
                 for (ConstValueIterator itr = v->Begin(); itr != v->End(); ++itr) {
                     typedef Hasher<EncodingType, MemoryPoolAllocator<> > EnumHasherType;
-                    char buffer[256 + 24];
+                    char buffer[256u + 24];
                     MemoryPoolAllocator<> hasherAllocator(buffer, sizeof(buffer));
                     EnumHasherType h(&hasherAllocator, 256);
                     itr->Accept(h);

test/unittest/schematest.cpp

@@ -1762,7 +1762,7 @@ private:
     typename DocumentType::AllocatorType documentAllocator_;
     typename SchemaDocumentType::AllocatorType schemaAllocator_;
     char documentBuffer_[16384];
-    char schemaBuffer_[128 * 1024];
+    char schemaBuffer_[128u * 1024];
 };
 
 TEST(SchemaValidator, TestSuite) {

test/unittest/simdtest.cpp

@@ -109,8 +109,8 @@ struct ScanCopyUnescapedStringHandler : BaseReaderHandler<UTF8<>, ScanCopyUnesca
 
 template <unsigned parseFlags, typename StreamType>
 void TestScanCopyUnescapedString() {
-    char buffer[1024 + 5 + 32];
-    char backup[1024 + 5 + 32];
+    char buffer[1024u + 5 + 32];
+    char backup[1024u + 5 + 32];
 
     // Test "ABCDABCD...\\"
     for (size_t offset = 0; offset < 32; offset++) {