Windows printing: handle failure of EnumPrinters.

A user reports that if the Print Spooler service is disabled via
services.msc, then PuTTY can report 'Out of memory!' when you try to
open the Terminal config pane, which is the one containing the combo
box enumerating the available printers.

Apparently this is because the call to EnumPrinters failed with the
error code other than the expected ERROR_INSUFFICIENT_BUFFER, and in
the process, left garbage in the pcbNeeded output parameter. That
wouldn't be too surprising if it had simply _not written_ to that
parameter and therefore it was never initialised at all in the calling
function printer_add_enum. But in fact, printer_add_enum *does*
precautionarily initialise needed=0 before the initial call to
EnumPrinters. So EnumPrinters must have actively written one of its
*own* uninitialised variables into it!

Anyway, the obvious fix is to distinguish ERROR_INSUFFICIENT_BUFFER
from any other kind of EnumPrinters failure (in fact turning off Print
Spooler seems to lead to RPC_S_SERVER_UNAVAILABLE), and not attempt to
proceed in the case of other failures.

windows/printing.c

@@ -67,11 +67,15 @@ static bool printer_add_enum(int param, DWORD level, char **buffer,
 
     /*
      * Exploratory call to EnumPrinters to determine how much space
-     * we'll need for the output. Discard the return value since it
-     * will almost certainly be a failure due to lack of space.
+     * we'll need for the output.
+     *
+     * If we get ERROR_INSUFFICIENT_BUFFER, that's fine, we're
+     * prepared to deal with it. Any other error, we return failure.
      */
-    p_EnumPrinters(param, NULL, level, (LPBYTE)((*buffer)+offset), 512,
-                   &needed, &nprinters);
+    if (p_EnumPrinters(param, NULL, level, (LPBYTE)((*buffer)+offset), 512,
+                       &needed, &nprinters) == 0 &&
+        GetLastError() != ERROR_INSUFFICIENT_BUFFER)
+        return false;
 
     if (needed < 512)
         needed = 512;