[libpng16] Added "/* SAFE */" comments in pngrutil.c and pngrtran.c

where warnings may be erroneously issued by code-checking applications.
Revised manual about changes in iTXt chunk handling made in libpng-1.6.0.
diff --git a/pngpread.c b/pngpread.c
index f132ce6..0169ecb 100644
--- a/pngpread.c
+++ b/pngpread.c
@@ -151,7 +151,7 @@
 void /* PRIVATE */
 png_push_read_sig(png_structrp png_ptr, png_inforp info_ptr)
 {
-   png_size_t num_checked = png_ptr->sig_bytes,
+   png_size_t num_checked = png_ptr->sig_bytes, /* SAFE, does not exceed 8 */ 
              num_to_check = 8 - num_checked;
 
    if (png_ptr->buffer_size < num_to_check)