fix assertion in compressed pctrace writer

src/plugin/pctrace.cpp

@@ -58,10 +58,9 @@ public:
 
 private:
     int_type overflow(int_type ch) override {
-        assert(pptr() > epptr());
+        compress_and_write();
         *pptr() = static_cast<char_type>(ch);
         pbump(1);
-        compress_and_write();
         return ch;
     }