osc: don't assert on \E]4 being followed by a ';'

This fixes a crash (in debug builds) that could be triggered by
e.g. cat:ing /dev/urandom.

osc.c

@@ -431,6 +431,9 @@ osc_dispatch(struct terminal *term)
         /* Set color<idx> */
 
         string--;
+        if (*string != ';')
+            break;
+
         assert(*string == ';');
 
         for (const char *s_idx = strtok(string, ";"), *s_color = strtok(NULL, ";");