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.

CHANGELOG.md

@@ -57,6 +57,8 @@
 * Sixel images being erased when printing text next to them.
 * Crash when last rendered cursor cell had scrolled off screen and
   `\E[J3` was executed.
+* Assert (debug builds) when an `\e]4` OSC escape was not followed by
+  a `;`.
 
 
 ### Security

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, ";");