[libpng16] Check for 0 return from png_get_rowbytes() in contrib/pngminus/*.c

to stop some Coverity issues (162705, 162706, and 162707).

ANNOUNCE

-Libpng 1.6.32beta11 - August 7, 2017
+Libpng 1.6.32beta12 - August 7, 2017
 
 This is not intended to be a public release. It will be replaced
 within a few weeks by a public version or by another test version.
@@ -8,20 +8,20 @@ Files available for download:
 Source files with LF line endings (for Unix/Linux) and with a
 "configure" script
 
-   1.6.32beta11.tar.xz (LZMA-compressed, recommended)
-   1.6.32beta11.tar.gz
+   1.6.32beta12.tar.xz (LZMA-compressed, recommended)
+   1.6.32beta12.tar.gz
 
 Source files with CRLF line endings (for Windows), without the
 "configure" script
 
-   lp1632b11.7z  (LZMA-compressed, recommended)
-   lp1632b11.zip
+   lp1632b12.7z  (LZMA-compressed, recommended)
+   lp1632b12.zip
 
 Other information:
 
-   1.6.32beta11-README.txt
-   1.6.32beta11-LICENSE.txt
-   libpng-1.6.32beta11-*.asc (armored detached GPG signatures)
+   1.6.32beta12-README.txt
+   1.6.32beta12-LICENSE.txt
+   libpng-1.6.32beta12-*.asc (armored detached GPG signatures)
 
 Changes since the last public release (1.6.31):
 
@@ -94,6 +94,12 @@ Version 1.6.32beta11 [August 7, 2017]
     after the keyword has been read.
   Check that the eXIf chunk has at least 2 bytes and begins with "II" or "MM".
 
+Version 1.6.32beta12 [August 7, 2017]
+  Added a set of "huge_xxxx_chunk.png" files to contrib/testpngs/crashers,
+    one for each known chunk type, with length = 2GB-1.
+  Check for 0 return from png_get_rowbytes() in contrib/pngminus/*.c to stop
+    some Coverity issues (162705, 162706, and 162707).
+
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit
 https://lists.sourceforge.net/lists/listinfo/png-mng-implement

CHANGES

@@ -5977,6 +5977,12 @@ Version 1.6.32beta11 [August 7, 2017]
     after the keyword has been read.
   Check that the eXIf chunk has at least 2 bytes and begins with "II" or "MM".
 
+Version 1.6.32beta12 [August 7, 2017]
+  Added a set of "huge_xxxx_chunk.png" files to contrib/testpngs/crashers,
+    one for each known chunk type, with length = 2GB-1.
+  Check for 0 return from png_get_rowbytes() in contrib/pngminus/*.c to stop
+    some Coverity issues (162705, 162706, and 162707).
+
 Send comments/corrections/commendations to png-mng-implement at lists.sf.net
 (subscription required; visit
 https://lists.sourceforge.net/lists/listinfo/png-mng-implement

contrib/pngminus/png2pnm.c

@@ -321,7 +321,7 @@ BOOL png2pnm (FILE *png_file, FILE *pnm_file, FILE *alpha_file,
   /* row_bytes is the width x number of channels x (bit-depth / 8) */
   row_bytes = png_get_rowbytes (png_ptr, info_ptr);
 
-  if (height > ((size_t)(-1))/row_bytes) /* too big */ {
+  if (!row_bytes || height > ((size_t)(-1))/row_bytes) /* too big */ {
     png_destroy_read_struct (&png_ptr, &info_ptr, NULL);
     return FALSE;
   }

contrib/pngminus/pnm2png.c

@@ -374,7 +374,7 @@ BOOL pnm2png (FILE *pnm_file, FILE *png_file, FILE *alpha_file, BOOL interlace,
     /* row_bytes is the width x number of channels x (bit-depth / 8) */
     row_bytes = width * channels * ((bit_depth <= 8) ? 1 : 2);
 
-  if (height > ((size_t)(-1))/row_bytes) /* too big */ {
+  if (!rowbytes || height > ((size_t)(-1))/row_bytes) /* too big */ {
     return FALSE;
   }
   if ((png_pixels = (png_byte *)