"UNADDRESSABLE ACCESS beyond top of stack" for 'large' stacks under x64

Using Dr. Memory version 2.1.0 -- build 1 on Windows 7 x64 on the following example
```c++
// gcc -o test.exe test.c
// drmemory -- test.exe
#include
#include

int main() {
char str[4049] = {0}; // using 4048 bytes instead reduces the error count to 2 from 3
puts(str);
return EXIT_SUCCESS;
}
```
gives me false errors (see [results.txt](https://github.com/DynamoRIO/drmemory/files/3104203/results.txt)). The example was compiled with [GCC 8.3.0](https://sourceforge.net/projects/gcc-win64/files/8.3.0/).