synchronize ape's vfprintf with libstdio

in ape's vfprintf we don't check if the file we're writing is actually a string buffer, resulting in a return of -1, when we should actually return the number of bytes that would be written.

sys/src/ape/lib/ap/stdio/vfprintf.c

@@ -202,7 +202,15 @@ vfprintf(FILE *f, const char *s, va_list args)
 			nprint++;
 		}
 	}
-	return ferror(f)? -1: nprint;;
+
+	if(ferror(f)){
+		if((f->flags&STRING) && f->wp==f->rp && f->wp>f->buf){
+			*(f->wp-1) = '\0';
+			return nprint;
+		}
+		return -1;
+	}
+	return nprint;
 }
 
 static int