Fix infinite loop in index("") (fix #1815)

src/jv.c

@@ -658,6 +658,8 @@ jv jv_string_indexes(jv j, jv k) {
   int idxlen = jv_string_length_bytes(jv_copy(k));
   jv a = jv_array();
 
+  if (idxlen == 0)
+    return a;
   p = jstr;
   while ((p = _jq_memmem(p, (jstr + jlen) - p, idxstr, idxlen)) != NULL) {
     a = jv_array_append(a, jv_number(p - jstr));

tests/jq.test

@@ -1538,4 +1538,9 @@ isempty(1,error("foo"))
 null
 false
 
+# Regression test for #1815
+index("")
+""
+null
+