Revert "Bug 790784 - (CVE-2017-17784) heap overread in gbr parser / load_image."

This reverts commit 06d24a79af.

The CVE is still fixed but now in a different way. Commit 4fa0cd4dcf
passes instead the accurate string length when using the string, hence
making it work even when not NUL-terminated. This has the advantage of
having the GBR file loaded in the end, despite such file format error. I
am personally not persuaded this is the best path since a file with such
an error may either be corrupted, or worse may have been constructed on
purpose to be harmful, so rejecting it directly may be the safe choice.
Nevertheless I may also be too doubtful and maybe trying to save a
slightly corrupted file may be the nicest choice indeed.

plug-ins/common/file-gbr.c

@@ -463,8 +463,7 @@ load_image (GFile   *file,
 
       if (! g_input_stream_read_all (input, temp, size,
                                      &bytes_read, NULL, error) ||
-          bytes_read != size                                   ||
-          temp[size - 1] != '\0')
+          bytes_read != size)
         {
           g_set_error (error, G_FILE_ERROR, G_FILE_ERROR_FAILED,
                        _("Error in GIMP brush file '%s'"),