fix line-buffered flush omission for odd usage of putc-family functions

as specified, the int argument providing the character to write is
converted to type unsigned char. for the actual write to buffer,
conversion happened implicitly via the assignment operator; however,
the logic to check whether the argument was a newline used the
original int value. thus usage such as putchar('\n'+0x100) failed to
produce a flush.

src/internal/stdio_impl.h

@@ -86,7 +86,8 @@ void __ofl_unlock(void);
 #define getc_unlocked(f) \
 	( ((f)->rpos < (f)->rend) ? *(f)->rpos++ : __uflow((f)) )
 
-#define putc_unlocked(c, f) ( ((c)!=(f)->lbf && (f)->wpos<(f)->wend) \
+#define putc_unlocked(c, f) \
+	( ((unsigned char)(c)!=(f)->lbf && (f)->wpos<(f)->wend) \
 	? *(f)->wpos++ = (c) : __overflow((f),(c)) )
 
 /* Caller-allocated FILE * operations */