input: check end of input before reading a byte

When reading bytes we weren't check that the index wasn't
out of bound and this could produce an invalid read which
could deal to a security bug.

Upstream-Status: Backport[https://gitlab.gnome.org/GNOME/libcroco/
                 commit/898e3a8c8c0314d2e6b106809a8e3e93cf9d4394]

CVE: CVE-2017-7960 

Signed-off-by: Changqing Li <changqing.li@windriver.com>

diff --git a/src/cr-input.c b/src/cr-input.c
index 49000b1f5f07fe057135f1b8fc69bdcf9613e300..3b63a88ee3b1c56778e58172d147d958951bf099 100644
--- a/src/cr-input.c
+++ b/src/cr-input.c
@@ -256,7 +256,7 @@ cr_input_new_from_uri (const gchar * a_file_uri, enum CREncoding a_enc)
                  *we should  free buf here because it's own by CRInput.
                  *(see the last parameter of cr_input_new_from_buf().
                  */
-                buf = NULL ;
+                buf = NULL;
         }
 
  cleanup:
@@ -404,6 +404,8 @@ cr_input_get_nb_bytes_left (CRInput const * a_this)
 enum CRStatus
 cr_input_read_byte (CRInput * a_this, guchar * a_byte)
 {
+        gulong nb_bytes_left = 0;
+
         g_return_val_if_fail (a_this && PRIVATE (a_this)
                               && a_byte, CR_BAD_PARAM_ERROR);
 
@@ -413,6 +415,12 @@ cr_input_read_byte (CRInput * a_this, guchar * a_byte)
         if (PRIVATE (a_this)->end_of_input == TRUE)
                 return CR_END_OF_INPUT_ERROR;
 
+        nb_bytes_left = cr_input_get_nb_bytes_left (a_this);
+
+        if (nb_bytes_left < 1) {
+                return CR_END_OF_INPUT_ERROR;
+        }
+
         *a_byte = PRIVATE (a_this)->in_buf[PRIVATE (a_this)->next_byte_index];
 
         if (PRIVATE (a_this)->nb_bytes -
@@ -477,7 +485,6 @@ cr_input_read_char (CRInput * a_this, guint32 * a_char)
                 if (*a_char == '\n') {
                         PRIVATE (a_this)->end_of_line = TRUE;
                 }
-
         }
 
         return status;