Allow whitespace at end of expression.

test.c

@@ -38,6 +38,7 @@ typedef struct {
 void test_results() {
     test_case cases[] = {
         {"1", 1},
+        {"1 ", 1},
         {"(1)", 1},
 
         {"pi", 3.14159},

tinyexpr.c

@@ -172,13 +172,13 @@ static double comma(double a, double b) {return b;}
 void next_token(state *s) {
     s->type = TOK_NULL;
 
-    if (!*s->next){
-        s->type = TOK_END;
-        return;
-    }
-
     do {
 
+        if (!*s->next){
+            s->type = TOK_END;
+            return;
+        }
+
         /* Try reading a number. */
         if ((s->next[0] >= '0' && s->next[0] <= '9') || s->next[0] == '.') {
             s->value = strtod(s->next, (char**)&s->next);