[deflate] Fix literal data length calculation

Fix incorrect calculation used to determine length of data to be
copied within a literal data block, and add a test case to prevent
this bug from going undetected in future.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/deflate.c

 		/* Calculate available amount of literal data */
 		in_remaining = ( in->len - in->offset );
 		len = deflate->remaining;
-		if ( len < in_remaining )
+		if ( len > in_remaining )
 			len = in_remaining;
 
 		/* Copy data to output buffer */

src/tests/deflate_test.c

 	  DATA ( 0x01, 0x04, 0x00, 0xfb, 0xff, 0x69, 0x50, 0x58, 0x45 ),
 	  DATA ( 0x69, 0x50, 0x58, 0x45 ) );
 
+/* "iPXE" string, no compression, split into two literals */
+DEFLATE ( split_literal, DEFLATE_RAW,
+	  DATA ( 0x00, 0x02, 0x00, 0xfd, 0xff, 0x69, 0x50, 0x01, 0x02, 0x00,
+		 0xfd, 0xff, 0x58, 0x45 ),
+	  DATA ( 0x69, 0x50, 0x58, 0x45 ) );
+
 /* Empty file */
 DEFLATE ( empty, DEFLATE_RAW, DATA ( 0x03, 0x00 ), DATA() );
 
 		/* Test as a single pass */
 		deflate_ok ( deflate, &empty_literal, NULL );
 		deflate_ok ( deflate, &literal, NULL );
+		deflate_ok ( deflate, &split_literal, NULL );
 		deflate_ok ( deflate, &empty, NULL );
 		deflate_ok ( deflate, &hello_world, NULL );
 		deflate_ok ( deflate, &hello_hello_world, NULL );