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[as] Add number parsing

Until this moment we were using atoi() to convert strings
to numbers. This patch adds a function that does this work
and it also verifies that there isn't any overflow.

--- a/as/expr.c

+++ b/as/expr.c

@@ -1,6 +1,7 @@

 static char sccsid[] = "@(#) ./as/node.c";

 #include <ctype.h>

+#include <limits.h>

 #include <stdlib.h>

 #include <string.h>

@@ -210,13 +211,27 @@

 static int

 number(void)

 {

-	int c;

+	int c, base = 10;

 	char *p;

+	TUINT n;

-	while (isxdigit(*endp))

+	if (*endp == '0') {

+		base = 8;

 		++endp;

+		if (*endp == 'x') {

+			base = 16;

+			++endp;

+		}

+	}

+	for (n = 0; (c = *endp) && isxdigit(c); n += c) {

+		n *= base;

+		c -= '0';

+		if (n >= TUINT_MAX - c*base)

+			error("overflow in number");

+		endp++;

+	}

 	tok2str();

-	yylval.sym = tmpsym(atoi(yytext));  /* TODO: parse the string */

+	yylval.sym = tmpsym(n);

 	return NUMBER;

 }