[flang] Fix excess allocation (#96663)

A recent patch introduced an error in an aligned byte size calculation
that causes an extra word to be allocated when the original byte size is
already aligned (including the case of zero). Fix.

Author: klausler

flang/runtime/pointer.cpp

@@ -127,7 +127,7 @@ void RTDEF(PointerAssociateRemapping)(Descriptor &pointer,
 RT_API_ATTRS void *AllocateValidatedPointerPayload(std::size_t byteSize) {
   // Add space for a footer to validate during deallocation.
   constexpr std::size_t align{sizeof(std::uintptr_t)};
-  byteSize = ((byteSize / align) + 1) * align;
+  byteSize = ((byteSize + align - 1) / align) * align;
   std::size_t total{byteSize + sizeof(std::uintptr_t)};
   void *p{std::malloc(total)};
   if (p) {
@@ -197,7 +197,7 @@ static RT_API_ATTRS std::size_t GetByteSize(
 bool RT_API_ATTRS ValidatePointerPayload(const ISO::CFI_cdesc_t &desc) {
   std::size_t byteSize{GetByteSize(desc)};
   constexpr std::size_t align{sizeof(std::uintptr_t)};
-  byteSize = ((byteSize / align) + 1) * align;
+  byteSize = ((byteSize + align - 1) / align) * align;
   const void *p{desc.base_addr};
   const std::uintptr_t *footer{reinterpret_cast<const std::uintptr_t *>(
       static_cast<const char *>(p) + byteSize)};