wasm: fix odd WASI readline behavior.

Most of the time, the WASI fd_read call returns the whole line including
the newline. Other times, it returns everything except the newline. So
before stompiing the newline, check for that condition. This fixes the
WASI based wasm implementations (wasmtime, wasmer, lucet).

impls/wasm/platform_wasi.wam

@@ -49,7 +49,12 @@
     (if (i32.le_s (i32.load $nread_ptr) 0)
       (return 0))
     ;; Replace ending newline with NULL
-    (i32.store8 (i32.add $buf (i32.sub (i32.load $nread_ptr) 1)) 0)
+    ;; NOTE: oddly, there isn't always a newline so check first
+    ;; Specifically, this input chops too much:
+    ;;   (abcd abcdefg (abc (n) (if (> n 0) (+ n (abcdefg  (- n 1))) 0)))
+    (if (i32.eq (CHR "\n")
+                (i32.load8_u (i32.add $buf (i32.sub (i32.load $nread_ptr) 1)) 0))
+      (i32.store8 (i32.add $buf (i32.sub (i32.load $nread_ptr) 1)) 0))
     1
   )