QTemporaryDir: fail early if the error isn't EEXIST

Before, stat -c on Linux (enabling this code path):

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 0.00    0.000000           0       256       256 mkdir

After:
 0.00    0.000000           0         1         1 mkdir

“To err is human, to persist in error is diabolical” - Georges
Canguilhem

“The definition of insanity is repeating the same mistakes over and over
again and expecting different results.” - Albert Einstein, Mark Twain or
Benjamin Franklin (all mis-attributed)

Change-Id: Ib306f8f647014b399b87ffff13f0a3c155053e6a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/io/qtemporarydir.cpp

@@ -135,6 +135,15 @@ QPair<QString, bool> q_mkdtemp(char *templateName)
             }
             return qMakePair(QFile::decodeName(templateName), true);
         }
+#  ifdef Q_OS_WIN
+        const int exists = ERROR_ALREADY_EXISTS;
+        int code = GetLastError();
+#  else
+        const int exists = EEXIST;
+        int code = errno;
+#  endif
+        if (code != exists)
+            return qMakePair(qt_error_string(code), false);
     }
     return qMakePair(qt_error_string(), false);
 }