fix tga header read

When you ping() an image with imagemagick to get the header, some format
loaders (eg. TGA) don't set all the fields. In this case,
image->colormap was not set and that made GetImageChannelDepth() crash.

Work around this, at least in this case, by allocating a colourmap
ourselves if none is set.

see https://github.com/jcupitt/libvips/issues/980

configure.ac

@@ -704,6 +704,17 @@ if test x"$magick6" = x"yes"; then
   LIBS="$save_LIBS"
 fi
 
+if test x"$magick6" = x"yes"; then
+  # more recent magick6s have AcquireImageColormap rather than 
+  # AllocateImageColormap groan
+  save_LIBS="$LIBS"
+  LIBS="$LIBS $MAGICK_LIBS"
+  AC_CHECK_FUNCS(AcquireImageColormap,
+    AC_DEFINE(HAVE_ACQUIREIMAGECOLORMAP,1,
+	      [define if your magick has AcquireImageColormap.]))
+  LIBS="$save_LIBS"
+fi
+
 if test x"$magick6" = x"yes"; then
   # more recent magick6s have SetImageExtent 
   save_LIBS="$LIBS"

libvips/foreign/magick.c

@@ -52,6 +52,12 @@ magick_acquire_image( const ImageInfo *image_info, ExceptionInfo *exception )
 	return( AcquireImage( image_info, exception ) );
 }
 
+void
+magick_acquire_image_colormap( Image *image, int colors )
+{
+	AcquireImageColormap( image, colors )
+}
+
 void
 magick_acquire_next_image( const ImageInfo *image_info, Image *image,
 	ExceptionInfo *exception )
@@ -107,6 +113,16 @@ magick_acquire_image( const ImageInfo *image_info, ExceptionInfo *exception )
 #endif
 }
 
+void
+magick_acquire_image_colormap( Image *image, int colors )
+{
+#ifdef HAVE_ACQUIREIMAGECOLORMAP
+	AcquireImageColormap( image, colors ); 
+#else /*!HAVE_ACQUIREIMAGECOLORMAP*/
+	AllocateImageColormap( image, colors ); 
+#endif /*HAVE_ACQUIREIMAGECOLORMAP*/
+}
+
 void
 magick_acquire_next_image( const ImageInfo *image_info, Image *image,
 	ExceptionInfo *exception )

libvips/foreign/magick.h

@@ -44,6 +44,7 @@
 
 Image *magick_acquire_image( const ImageInfo *image_info, 
 	ExceptionInfo *exception );
+void magick_acquire_image_colormap( Image *image, int colors );
 void magick_acquire_next_image( const ImageInfo *image_info, 
 	Image *image, ExceptionInfo *exception );
 int magick_set_image_size( Image *image, 

libvips/foreign/magick2vips.c

@@ -57,6 +57,8 @@
  * 	- try using GetImageChannelDepth() instead of ->depth
  * 24/4/18
  * 	- add format hint
+ * 25/5/18
+ * 	- acquire an image colormap if none set
  */
 
 /*
@@ -327,6 +329,15 @@ parse_header( Read *read )
 	if( (im->Bands = get_bands( image )) < 0 )
 		return( -1 );
 
+	/* Some ImageMagick loaders (eg. TGA) fail to set the ->colormap
+	 * field on Ping. GetImageChannelDepth() needs this and
+	 * will crash if it's not set.
+	 *
+	 * If there's no colormap, set an empty one.
+	 */
+	if( !image->colormap ) 
+		magick_acquire_image_colormap( image, image->colors );
+
 	/* Depth can be 'fractional'. 
 	 *
 	 * You'd think we should use