fix carry into uninitialized slots during printf floating point rounding

in cases where rounding caused a carry, the slot into which the carry
was taking place was unconditionally treated as valid, despite the
possibility that it could be a new slot prior to the beginning of the
existing non-rounded number. in theory this could lead to unbounded
runaway carry, but in order for that to happen, the whole
uninitialized buffer would need to have been pre-filled with 32-bit
integer values greater than or equal to 999999999.

patch based on proposed fix by Morten Welinder, who also discovered
and reported the bug.

(cherry picked from commit 109048e031f39fbb370211fde44ababf6c04c8fb)

src/stdio/vfprintf.c

@@ -356,9 +356,9 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
 				*d = *d + i;
 				while (*d > 999999999) {
 					*d--=0;
+					if (d<a) *--a=0;
 					(*d)++;
 				}
-				if (d<a) a=d;
 				for (i=10, e=9*(r-a); *a>=i; i*=10, e++);
 			}
 		}