Bugfix #4661

Apparently the atof() function allows to convert a NAN string input to a
NAN float value.
That we don't want when you input values in our sliders! :)

source/blender/src/interface.c

@@ -2051,7 +2051,9 @@ static int ui_act_as_text_but(uiBut *but)
 		}
 	}
 	else value = atof(str);
-
+	/* NAN detection */
+	if(isnan(value)) value= 0.0;
+	
 	if(but->pointype!=FLO) value= (int)value;
 
 	if(value<min) value= min;