Fix. Try to reduce while converting to float

Some times if you are performing a lot of calculations, you'll get a big integer inside decimal tuple with a lot of zeros like this one.
``` erlang
{0,
1002760000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000,
 -346}
```
If, you'll try to add something to this decimal, you'll get an error:
```
** exception error: an error occurred when evaluating an arithmetic expression
     in function  decimal_conv:float/1 (src/decimal_conv.erl, line 8)
     in call from decimal_arith:add_sign/4 (src/decimal_arith.erl, line 64)
     in call from decimal_arith:add/3 (src/decimal_arith.erl, line 30)
```
In this case i'ts better to reduce it and try again.