Bring minimum precision inline with spprintf

The precision "minimum" for spprintf was changed in
3f23e6bca9 with the cryptic comment "Enable 0
mode for echo/print". Since then the behaviour of spprintf and snprintf has not
been the same. This results in some APIs handling precision differently than
others, which then resulted in the following Xdebug issue:
https://bugs.xdebug.org/view.php?id=2151

The "manpage" for snprinf says about precision:

       An optional precision, in the form of a period ('.')   followed  by  an
       optional  decimal  digit string.  Instead of a decimal digit string one
       may write "*" or "*m$" (for some decimal integer m) to specify that the
       precision  is  given in the next argument, or in the m-th argument, re‐
       spectively, which must be of type int.  If the precision  is  given  as
       just  '.',  the precision is taken to be zero.  A negative precision is
       taken as if the precision were omitted.

However, the snprintf implementation never supported this "negative precision",
which is what PHP's default setting is in PG(precision). However, in
3f23e6bca9 spprintf was made to support this.

Although this techinically can break BC, there is clearly a bug here, and I
could not see any failing tests locally.

main/snprintf.c

@@ -580,8 +580,8 @@ static size_t format_converter(buffy * odp, const char *fmt, va_list ap) /* {{{
 					} else if (*fmt == '*') {
 						precision = va_arg(ap, int);
 						fmt++;
-						if (precision < 0)
-							precision = 0;
+						if (precision < -1)
+							precision = -1;
 					} else
 						precision = 0;
 				} else