MiNTlib: make setvbuf more robust

[Andreas Schwab <schwab@issan.informatik.uni-dortmund.de>]

This patch makes the function setvbuf more robust by checking the
bmode parameter.

--- orig/setvbuf.c	Sun Sep  6 05:00:32 1992
+++ setvbuf.c	Sat Aug 27 17:13:12 1994
@@ -10,6 +10,8 @@
 int bmode;
 size_t size;
 {
+    if (bmode != _IOFBF && bmode != _IOLBF && bmode != _IONBF)
+      return -1;
     if(fp->_flag & _IOMYBUF)
 	free(fp->_base);
     fp->_flag &= ~(_IOFBF | _IOLBF | _IONBF | _IOMYBUF);