Fix useless assert(true) (which would never fire)

...and simplify the code a bit.  It had been introduced with
0a1d5af2a1 "tdf#115429 sax: assert if exporting an
invalid XML attribute/element", apparently meant to be an assert(false) instead.
A `make check screenshot` didn't cause the (now active) assert to fire in my
local build.

Change-Id: I275661f290491ff2d094288522f98d77cb4662ec
Reviewed-on: https://gerrit.libreoffice.org/82631
Reviewed-by: Michael Stahl <michael.stahl@cib.de>
Tested-by: Jenkins

sax/source/expatwrap/saxwriter.cxx

@@ -578,10 +578,8 @@ void CheckValidName(OUString const& rName)
         auto const c(rName[i]);
         if (c == ':')
         {
-            if (hasColon)
-                assert("only one colon allowed");
-            else
-                hasColon = true;
+            assert(!hasColon && "only one colon allowed");
+            hasColon = true;
         }
         else if (!rtl::isAsciiAlphanumeric(c) && c != '_' && c != '-' && c != '.' &&
                  !inrange(c, 0x00C0, 0x00D6) && !inrange(c, 0x00D8, 0x00F6) &&