That's caused by a bug in the Solaris 2.4 named. You need to install
the appropriate one of the following patches:
102479-01: SunOS 5.4: memory leak/mismanagement in in.named
102480-01: SunOS 5.4_x86: memory leak/mismanagement in in.named
The latest version of these patches include bind 4.9.3:
102479-12: SunOS 5.4: libresolv, in.named, named-xfer, nslookup ...
102480-10: SunOS 5.4_x86: libresolv, in.named, named-xfer, nslookup ...