Fix incorrect error on read with 1-character input

When `read` is given a single character of input (including the
newline), it was bailing as if it had been given no input. This is
incorrect.

tests/read.in

@@ -38,6 +38,8 @@ echo 'test' | read -l one two three
 print_vars one two three
 echo 'foo   bar   baz' | read -l one two three
 print_vars one two three
+echo -n 'a' | read -l one
+echo "$status $one"
 
 echo
 set -l IFS

tests/read.out

@@ -17,6 +17,7 @@ two
 1 '' 1 ''
 1 'test' 1 '' 1 ''
 1 'foo' 1 'bar' 1 '  baz'
+0 a
 
 1 'hello'
 1 'h' 1 'ello'