Cygwin: dladdr: use proper max size of dli_fname.

The DL_info::dli_fname member is actually PATH_MAX bytes, so specify
that (larger) size to cygwin_conv_path rather than MAX_PATH.

Also, use a tmp_pathbuf for the GetModuleFileNameW buffer, so that any
buffer size limitation will definitely be due to the size of dli_fname,
and add a static_assert of the size of dli_fname so we can be sure we're
using the right size constant here.

Fixes: c8432a0 ("Implement dladdr() (partially)")
Addresses: rust-lang/backtrace-rs#704 (comment)
Signed-off-by: Jeremy Drake <[email protected]>
(cherry picked from commit 38772dd)

Author: Jeremy Drake via Cygwin-patches

winsup/cygwin/dlfcn.cc

@@ -421,14 +421,16 @@ dladdr (const void *addr, Dl_info *info)
   /* Get the module filename.  This pathname may be in short-, long- or //?/
      format, depending on how it was specified when loaded, but we assume this
      is always an absolute pathname. */
-  WCHAR fname[MAX_PATH];
-  DWORD length = GetModuleFileNameW (hModule, fname, MAX_PATH);
-  if ((length == 0) || (length == MAX_PATH))
+  tmp_pathbuf tp;
+  PWCHAR fname = tp.w_get ();
+  DWORD length = GetModuleFileNameW (hModule, fname, NT_MAX_PATH);
+  if ((length == 0) || (length == NT_MAX_PATH))
     return 0;
 
   /* Convert to a cygwin pathname */
+  static_assert (sizeof (info->dli_fname) == PATH_MAX);
   ssize_t conv = cygwin_conv_path (CCP_WIN_W_TO_POSIX | CCP_ABSOLUTE, fname,
-				   info->dli_fname, MAX_PATH);
+				   info->dli_fname, PATH_MAX);
   if (conv)
     return 0;