win32/msvc cc does not recognize the %z format specifier which caused
invalid references and segfaults on win32.
Until FreeRDP gets format specifier macros we'll cast size_t to
unsigned long and use the %lu specifier.
Also simplified winpr_backtrace_symbols() a little bit and fixed it
to allocate the correct amount of bytes for the return buffer.
e8c4910e2e