FindWin32CACert: Use a temporary buffer on the stack

Don't malloc() the temporary buffer, and use the correct type:
SearchPath() works with TCHAR, but SearchPathA() works with char.
Set the buffer size to MAX_PATH, because the terminating null byte
is already included in MAX_PATH.

Reviewed-by: Daniel Stenberg
Reviewed-by: Marcel Raad

Closes #1548