Fix a few cppcheck issues

Found the following:

* Potential stack explosions with alloca() usage on potentially large
strings
* Memory leaks in WriteEngineServer
* Stack usage out of scope in dataconvert
* A typo in an 'if' statement in dataconvert

utils/funcexp/func_char_length.cpp

@@ -83,9 +83,9 @@ int64_t Func_char_length::getIntVal(rowgroup::Row& row,
                 return 0;
 
             size_t strwclen = utf8::idb_mbstowcs(0, tstr.c_str(), 0) + 1;
-            wchar_t* wcbuf = (wchar_t*)alloca(strwclen * sizeof(wchar_t));
+            wchar_t* wcbuf = new wchar_t[strwclen];
             strwclen = utf8::idb_mbstowcs(wcbuf, tstr.c_str(), strwclen);
-
+            delete [] wcbuf;
             return (int64_t)strwclen;
         }