Prevent garbage on DNS_TXT records

Bug #64458 fixed an off by one error when decoding TXT records
that resulted with garbage at the end of the string returned.

Reviewed By: @scannell

Differential Revision: D1146140

hphp/runtime/ext/ext_network.cpp

@@ -500,7 +500,7 @@ static unsigned char *php_parserr(unsigned char *cp, querybuf *answer,
       memcpy(tp + ll , cp + ll + 1, n);
       ll = ll + n + 1;
     }
-    s.setSize(dlen);
+    s.setSize(dlen > 0 ? dlen - 1 : 0);
     cp += dlen;
 
     subarray.set(s_txt, s);