Fix typo in decode_utf8 tests.

The test in question was supposed to contain the spurious UTF-8
encoding that 0xD800 would have if it were not a surrogate. But the
final continuation character 0x80 was instead 0x00.

The test passed anyway, because ED A0 was regarded as a truncated
sequence, instead of ED A0 80 being regarded as an illegal encoding of
a surrogate, and both return the same output!

utils/decode_utf8.c

@@ -128,7 +128,7 @@ int main(void)
 
     /* Endpoints of the surrogate range */
     DOTEST("\xED\x9F\xBF", 0xD7FF);
-    DOTEST("\xED\xA0\x00", 0xFFFD);    /* would be 0xD800 */
+    DOTEST("\xED\xA0\x80", 0xFFFD);    /* would be 0xD800 */
     DOTEST("\xED\xBF\xBF", 0xFFFD);    /* would be 0xDFFF */
     DOTEST("\xEE\x80\x80", 0xE000);