Tighten string buffer size calculation in enc_string.

When "\u"-escaping a Unicode character, the esc_extra value doesn't
need to include the number of bytes in the input string.  That is, if
a three-byte UTF-8 character is being escaped to a six-byte "\uXXXX"
sequence, esc_extra only needs to be increased by 3.

c_src/encoder.c

@@ -307,10 +307,7 @@ enc_string(Encoder* e, ERL_NIF_TERM val)
                     if(uval < 0) {
                         return 0;
                     }
-                    esc_extra += utf8_esc_len(uval);
-                    if(ulen < 0) {
-                        return 0;
-                    }
+                    esc_extra += utf8_esc_len(uval) - ulen;
                 }
                 i += ulen;
         }