Fix bug in fread

Bug discovered by @xTibor. Test and input data provided by him.

tests/stdio/fread.c

@@ -0,0 +1,21 @@
+#include <errno.h>
+#include <stdio.h>
+
+int main(int argc, char *argv[]) {
+    FILE *fp = fopen("stdio/fread.in", "rb");
+
+    char buf[33] = { 0 };
+    for (int i = 1; i <= 32; ++i) {
+        if (fread(buf, 1, i, fp) < 0) {
+            perror("fread");
+            return 0;
+        }
+        buf[i] = 0;
+
+        printf("%s\n", buf);
+    }
+
+    fclose(fp);
+
+    return 0;
+}

tests/stdio/fread.in

@@ -0,0 +1 @@
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