Avoid dereferencing before the start of a string.

A check for a backslash before a quote neglected to pretest that the
quote wasn't the first character in its string.

Change-Id: Ib5226836d1111e37bed984938f7c667be59eb1c5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qdatetimeparser.cpp

@@ -382,7 +382,7 @@ bool QDateTimeParser::parseFormat(const QString &newFormat)
             ++add;
             if (status != quote) {
                 status = quote;
-            } else if (newFormat.at(i - 1) != slash) {
+            } else if (i > 0 && newFormat.at(i - 1) != slash) {
                 status = zero;
             }
         } else if (status != quote) {