Make RGBDS behave identically whether writing a .o

Some errors are only tripped in `out_WriteObject`, which was
basically a stub when `-o` wasn't specified. Now, instead,
errors are checked in a separate function before out_WriteFile

src/asm/main.c

@@ -477,6 +477,10 @@ int main(int argc, char *argv[])
 			printf("(%d lines/minute)\n",
 			       (int)(60 / timespent * nTotalLines));
 	}
-	out_WriteObject();
+
+	out_CheckErrors();
+	/* If no path specified, don't write file */
+	if (tzObjectname != NULL)
+		out_WriteObject();
 	return 0;
 }