disallow invalid name characters

This commit makes using invalid characters in the name a fatal error.
Before this patch, "foo | bar(1)" would parse as "foobar(1)". Now it is
a fatal error and parsing stops.

src/main.c

@@ -138,6 +138,8 @@ static void parse_preamble(struct parser *p) {
 			}
 			fprintf(p->output, "\n");
 			break;
+		} else {
+			parser_fatal(p, "Name characters must be A-Z, a-z, 0-9, `-`, `_`, or `.`");
 		}
 	}
 	str_free(name);