fmemopen: allow reading NUL bytes

Previously a memory stream opened in 'r' mode would return EOF once it
encounters the first NUL byte but this is not correct according to the
POSIX man page. Now that we track size (occurrence of first NUL byte in
a R/W buffer) and bufsize separately we can return EOF once we hit
the current size on reads rather than checking for NUL.

Author: arichardson

newlib/libc/tinystdio/fmemopen.c

@@ -73,14 +73,13 @@ static int
 __fmem_get(FILE *f)
 {
     struct __file_mem *mf = (struct __file_mem *)f;
-    int c;
-    if ((f->flags & __SRD) && mf->pos < mf->size) {
-        c = (unsigned char)mf->buf[mf->pos++];
-        if (c == '\0')
-            c = _FDEV_EOF;
-    } else
-        c = _FDEV_ERR;
-    return c;
+    if ((f->flags & __SRD) == 0) {
+        return _FDEV_ERR;
+    } else if (mf->pos < mf->size) {
+        return (unsigned char)mf->buf[mf->pos++];
+    } else {
+        return _FDEV_EOF;
+    }
 }
 
 static int