IMPORT: slz: declare len to fix debug build when optimal match is enabled

Building with -DFIND_OPTIMAL_MATCH would fail on undeclared "len".
This one likely vanished in some cleanup.

This is libslz upstream commit 1ea20360715e1ad0cd81db83fa4361310716b8cc

src/slz.c

@@ -578,6 +578,8 @@ long slz_rfc1951_encode(struct slz_stream *strm, unsigned char *out, const unsig
 		int max_lookup = 2; // 0 = no limit
 
 		for (scan = pos - 1; scan < pos && (unsigned long)(pos - scan - 1) < 32768; scan--) {
+			int len;
+
 			if (*(uint32_t *)(in + scan) != word)
 				continue;