Return non-negative numbers.
 [rsc] --rw-rw-r-- M 73506 glenda sys 11556 Jan  7 11:35 sys/src/cmd/pic/input.c
	/n/sourcesdump/2006/0107/plan9/sys/src/cmd/pic/input.c:245,251 - 
	/n/sourcesdump/2006/0108/plan9/sys/src/cmd/pic/input.c:245,251
	  		printf(" <%c>", c);
	  	if (ep >= ebuf + sizeof ebuf)
	  		ep = ebuf;
	- 	return *ep++ = c;
	+ 	return *(unsigned char *)ep++ = c;
	  }
	  
	  nextchar(void)