fix spurious failures by fgetws when buffer ends with partial character

commit a90d9da1d1 made fgetws look for
changes to errno by fgetwc to detect encoding errors, since ISO C did
not allow the implementation to set the stream's error flag in this
case, and the fgetwc interface did not admit any other way to detect
the error. however, the possibility of fgetwc setting errno to EILSEQ
in the success path was overlooked, and in fact this can happen if the
buffer ends with a partial character, causing mbtowc to be called with
only part of the character available.

since that change was made, the C standard was amended to specify that
fgetwc set the stream error flag on encoding errors, and commit
511d70738b made it do so. thus, there is
no longer any need for fgetws to poke at errno to handle encoding
errors.

this commit reverts commit a90d9da1d1
and thereby fixes the problem.

src/stdio/fgetws.c

@@ -1,6 +1,5 @@
 #include "stdio_impl.h"
 #include <wchar.h>
-#include <errno.h>
 
 wint_t __fgetwc_unlocked(FILE *);
 
@@ -12,10 +11,6 @@ wchar_t *fgetws(wchar_t *restrict s, int n, FILE *restrict f)
 
 	FLOCK(f);
 
-	/* Setup a dummy errno so we can detect EILSEQ. This is
-	 * the only way to catch encoding errors in the form of a
-	 * partial character just before EOF. */
-	errno = EAGAIN;
 	for (; n; n--) {
 		wint_t c = __fgetwc_unlocked(f);
 		if (c == WEOF) break;
@@ -23,7 +18,7 @@ wchar_t *fgetws(wchar_t *restrict s, int n, FILE *restrict f)
 		if (c == '\n') break;
 	}
 	*p = 0;
-	if (ferror(f) || errno==EILSEQ) p = s;
+	if (ferror(f)) p = s;
 
 	FUNLOCK(f);