Problems with zero precision

There are some problems with a precision of 0 (an annoying corner case).

The following crash:

• printf("% .0d", 0); (should output )
• printf("%+.0d", 0); (should output +)

This is caused by decrementing a 0 len here: https://github.com/mpaland/printf/blob/master/printf.c#L185

printf("%#.0x", 0); is apparently supposed to output an empty string, not 0x. The octal case is correct, though. It seems this was clarified in C11.

[mpaland]

Wow! This is really serious and leads to out of bounds buffer access!
Wondering that coverity didn't catch it.

[eyalroz]

The fix for this bug may have causes #109 .