Always initialize m_format in DetermineDigits()

It doesn't seem right to leave it unchanged when increment is outside of
[0, 1] interval, we should still set it to something in this case.

And doing this makes it unnecessary and redundant to initialize m_format
in Init(), as it will be always done when DetermineDigits() is called
from Create() anyhow.

include/wx/generic/spinctlg.h

@@ -424,7 +424,6 @@ private:
     void Init()
     {
         m_digits = 0;
-        m_format = wxASCII_STR("%0.0f");
     }
 
     // Update m_digits and m_format to correspond to the given increment.

src/generic/spinctlg.cpp

@@ -778,8 +778,13 @@ void wxSpinCtrlDouble::DetermineDigits(double inc)
     if ( inc > 0.0 && inc < 1.0 )
     {
         m_digits = wxMin(SPINCTRLDBL_MAX_DIGITS, -static_cast<int>(floor(log10(inc))));
-        m_format.Printf("%%0.%ulf", m_digits);
     }
+    else
+    {
+        m_digits = 0;
+    }
+
+    m_format.Printf("%%0.%ulf", m_digits);
 }
 
 #endif // wxUSE_SPINBTN