Guard against invalid sequences from external matchfinders (#3465)

tests/external_matchfinder.h

@@ -20,7 +20,11 @@ typedef enum {
     EMF_ONE_BIG_SEQ = 1,
     EMF_LOTS_OF_SEQS = 2,
     EMF_BIG_ERROR = 3,
-    EMF_SMALL_ERROR = 4
+    EMF_SMALL_ERROR = 4,
+    EMF_INVALID_OFFSET = 5,
+    EMF_INVALID_MATCHLEN = 6,
+    EMF_INVALID_LITLEN = 7,
+    EMF_INVALID_LAST_LITS = 8
 } EMF_testCase;
 
 size_t zstreamExternalMatchFinder(