gd2: fix double free when processing invalid headers

This closes #208.

src/gd_gd2.c

@@ -359,9 +359,8 @@ BGD_DECLARE(gdImagePtr) gdImageCreateFromGd2Ctx (gdIOCtxPtr in)
 	im =
 	    _gd2CreateFromFile (in, &sx, &sy, &cs, &vers, &fmt, &ncx, &ncy,
 	                        &chunkIdx);
-
 	if (im == NULL) {
-		gdFree (chunkIdx);
+		/* No need to free chunkIdx as _gd2CreateFromFile does it for us. */
 		return 0;
 	}
 

tests/gd2/Makemodule.am

@@ -4,6 +4,8 @@ libgd_helper_programs += \
 
 libgd_test_programs += \
 	gd2/gd2_empty_file
+TESTS += \
+	gd2/invalid_header.sh
 
 if HAVE_LIBZ
 libgd_test_programs += \

tests/gd2/invalid_header.sh

@@ -0,0 +1,4 @@
+#!/bin/sh -x
+base=${0%.sh}
+input="${base}.gd2"
+exec ./gd2/gd2_read_corrupt "${input}"