fix a TGA ping crash

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

ChangeLog

@@ -3,6 +3,7 @@
 - revise C++ example [fangqiao]
 - strict round down on jpeg shrink on load [davidwood]
 - configure test for g++ 7.2 and composite.cpp
+- fix a crash with TGA ping [jtorresfabra]
 
 12/2/18 started 8.6.3
 - use pkg-config to find libjpeg, if we can

configure.ac

@@ -662,6 +662,17 @@ 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"$magick6" = x"yes"; then
   # more recent magicks have GetVirtualPixels rather than GetImagePixels
   save_LIBS="$LIBS"

libvips/foreign/magick2vips.c

@@ -335,6 +335,19 @@ 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 ) 
+#ifdef HAVE_ACQUIREIMAGECOLORMAP
+		AcquireImageColormap( image, image->colors ); 
+#else /*!HAVE_ACQUIREIMAGECOLORMAP*/
+		AllocateImageColormap( image, image->colors ); 
+#endif /*HAVE_ACQUIREIMAGECOLORMAP*/
+
 	/* Depth can be 'fractional'. 
 	 *
 	 * You'd think we should use