fix amazon-ion#221 - Ion binary floating point exception

The global decContext `g_IonEventDecimalContext` has traps set to
DEC_Errors, which wouldn't be set if `decContextDefault()` would be
used with any `kind` but DEC_INIT_BASE.

tools/events/ion_event_util.cpp

@@ -22,7 +22,7 @@ decContext g_IonEventDecimalContext = {
         DEC_MAX_MATH,                   // max exponent
         -DEC_MAX_MATH,                  // min exponent
         DEC_ROUND_HALF_EVEN,            // rounding mode
-        DEC_Errors,                     // trap conditions
+        0,                              // trap conditions
         0,                              // status flags
         0                               // apply exponent clamp?
 };