fix: non-integers passed to str-slice

See sass/sass-spec#1443
Closes sass#2922

src/fn_strings.cpp

@@ -163,6 +163,11 @@ namespace Sass {
         String_Constant* s = ARG("$string", String_Constant);
         double start_at = ARGVAL("$start-at");
         double end_at = ARGVAL("$end-at");
+
+        if (start_at != (int)start_at) {
+          error("$start-at: " + std::to_string(start_at) + " is not an int", pstate, traces);
+        }
+
         String_Quoted* ss = Cast<String_Quoted>(s);
 
         std::string str(s->value());
@@ -173,6 +178,10 @@ namespace Sass {
           end_at = -1;
         }
 
+        if (end_at != (int)end_at) {
+          error("$end-at: " + std::to_string(end_at) + " is not an int", pstate, traces);
+        }
+
         if (end_at == 0 || (end_at + size) < 0) {
           if (ss && ss->quote_mark()) newstr = quote("");
           return SASS_MEMORY_NEW(String_Quoted, pstate, newstr);