Remove an unnecessary variable assignment.

Found using Clang's static analyzer.

Python/pystrtod.c

@@ -1005,7 +1005,6 @@ format_float_short(double d, char format_code,
 			/* shouldn't get here: Gay's code should always return
 			   something starting with a digit, an 'I',  or 'N' */
 			strncpy(p, "ERR", 3);
-			p += 3;
 			assert(0);
 		}
 		goto exit;