In __mbsconv(), if prec was zero, nconv could have been used

uninitialized.  Initialize it to a safe value so that there's no
chance of returning an error if stack garbage happens to be equal to
(size_t)-1 or (size_t)-2.

Found by:	Clang static analyzer
MFC after:	7 days

lib/libc/stdio/vfwprintf.c

@@ -293,7 +293,7 @@ __mbsconv(char *mbsarg, int prec)
 		 * number of characters to print.
 		 */
 		p = mbsarg;
-		insize = nchars = 0;
+		insize = nchars = nconv = 0;
 		mbs = initial_mbs;
 		while (nchars != (size_t)prec) {
 			nconv = mbrlen(p, MB_CUR_MAX, &mbs);