try adding a file fuzzer

oss-fuzz normally fuzzes with memory buffers, but a lot of the libvips
loadres, especially CSV and PPM, only work from the filesystem.

This fuzzer writes the oss-fuzz buffer to the FS, then fuzzes that.

fuzz/Makefile.am

@@ -2,6 +2,7 @@ TESTS = \
 	test_fuzz.sh
 
 FUZZPROGS = \
+	jpegsave_file_fuzzer \
 	jpegsave_buffer_fuzzer \
 	pngsave_buffer_fuzzer \
 	webpsave_buffer_fuzzer \

fuzz/jpegsave_file_fuzzer.cc

@@ -0,0 +1,62 @@
+#include <vips/vips.h>
+
+extern "C" int
+LLVMFuzzerInitialize( int *argc, char ***argv )
+{
+	vips_concurrency_set( 1 );
+	return( 0 );
+}
+
+static int
+test_one_file( const char *name )
+{
+	VipsImage *image;
+	void *buf;
+	size_t len;
+
+	if( !(image = vips_image_new_from_file( name, 
+		"access", VIPS_ACCESS_SEQUENTIAL, 
+		NULL )) )
+		return( 0 );
+
+	/* Skip big images. They are likely to timeout.
+	 */
+	if( image->Xsize > 1024 ||
+		image->Ysize > 1024 ||
+		image->Bands > 10 ) {
+		g_object_unref( image );
+		return( 0 );
+	}
+
+	if( vips_jpegsave_buffer( image, &buf, &len, NULL ) ) {
+		g_object_unref( image );
+		return( 0 );
+	}
+
+	g_free( buf );
+	g_object_unref( image );
+
+	return( 0 );
+}
+
+extern "C" int
+LLVMFuzzerTestOneInput( const guint8 *data, size_t size )
+{
+	char *name;
+
+	if( !(name = vips__temp_name( "%s" )) )
+		return( 0 );
+
+	if( !g_file_set_contents( name, (const char *) data, size, NULL ) ||
+		test_one_file( name ) ) {
+		g_unlink( name );
+		g_free( name );
+
+		return( 0 );
+	}
+
+	g_unlink( name );
+	g_free( name );
+
+	return( 0 );
+}