Fixed output of gzip signature with NULL dates

src/binwalk/magic/compressed

@@ -137,7 +137,7 @@
 >3      byte            &0x20           \b, encrypted{invalid}
 # Dates before 1992 are {invalid}, unless of course you're DD-WRT in which
 # case you don't know how to set a date in your gzip files. Brilliant.
->4      lelong          =0              \b, NULL date:
+>4      lelong          =0              \b, NULL date (1970-01-01 00:00:00)
 >4      lelong          <0              {invalid}
 >4      lelong          >0
 >>4     lelong          <694224000      {invalid}