remove some no-op end of string tests from regex parser

these are cruft from the original code which used an explicit string
length rather than null termination. i blindly converted all the
checks to null terminator checks, without noticing that in several
cases, the subsequent switch statement would automatically handle the
null byte correctly.

src/regex/regcomp.c

@@ -1052,8 +1052,6 @@ tre_parse(tre_parse_ctx_t *ctx)
 	  }
 
 	case PARSE_UNION:
-	  if (!*ctx->re)
-	    break;
 	  switch (*ctx->re)
 	    {
 	    case CHAR_PIPE:
@@ -1086,8 +1084,6 @@ tre_parse(tre_parse_ctx_t *ctx)
 
 	case PARSE_POSTFIX:
 	  /* Parse postfix operators. */
-	  if (!*ctx->re)
-	    break;
 	  switch (*ctx->re)
 	    {
 	    case CHAR_PLUS: