stream_getc: use lib_stream_eof instead of EOF

Will case scanftest break #14778, at " %4c%n" case.
scanf use INT_MAX cause EOF break.

Signed-off-by: buxiasen <buxiasen@xiaomi.com>

libs/libc/stdio/lib_libvscanf.c

@@ -1149,7 +1149,7 @@ static int vscanf_internal(FAR struct lib_instream_s *stream, FAR int *lastc,
                 {
                   size_t nchars = (size_t) (stream->nget - ngetstart);
 
-                  if (c != EOF)
+                  if (!lib_stream_eof(c))
                     {
                       /* One more character already read */
 

libs/libc/stdio/lib_vsscanf.c

@@ -41,7 +41,7 @@ int vsscanf(FAR const char *buf, FAR const IPTR char *fmt, va_list ap)
 
   /* Initialize a memory stream to freadm from the buffer */
 
-  lib_meminstream(&meminstream, buf, INT_MAX);
+  lib_meminstream(&meminstream, buf, strlen(buf));
 
   /* Then let lib_vscanf do the real work */