Fix crash when sprintf width argument overflows u64

Given "printf %18446744073709551616s", we parse the number only in
the printf crate, which tells us that we overflowed somwhere (but
not where exactly).

tests/checks/printf.fish

@@ -151,3 +151,6 @@ echo
 # This is how mc likes to encode the directory we should cd to.
 printf '%b\n' '\0057foo\0057bar\0057'
 # CHECK: /foo/bar/
+
+printf %18446744073709551616s
+# CHECKERR: Number out of range