Fix UB in macro.c and streval.c

Clang with UBSan on macOS found these instances of undefined
behaviour. These two files both contain the test

	(Sflong_t)d!=d

to check if the value of a long double (Sfdouble_t) 'd' can be
represented in a variable of type long long (Sflong_t).

This test has undefined behaviour if isinf(d), if isnan(d), or if
the value of d is smaller or greater than the minimum or maximum
value that can be stored in an Sflong_t variable.

So this commit adds tests for those conditions. The original test,
(Sflong_t)d!=d, should now only be used to figure out if there is a
decimal fraction, which should be well-defined behaviour.

Note that LDBL_LLONG_MIN and LDBL_LLONG_MAX are defined in
streval.h; they are LLONG_MIN and LLONG_MAX typecast to Sfdouble_t.

Author: McDutchie

src/cmd/ksh93/sh/macro.c

@@ -2210,7 +2210,8 @@ static void comsubst(Mac_t *mp,Shnode_t* t, char type)
 		out_offset:
 			stkset(stkp,savptr,savtop);
 			*mp = savemac;
-			if((Sflong_t)num!=num)
+			/* must use <= >= not < >, as we have reduced precision at these sizes */
+			if(isinf(num) || isnan(num) || num <= LDBL_LLONG_MIN || num >= LDBL_LLONG_MAX || (Sflong_t)num!=num)
 				sfprintf(sh.strbuf,"%.*Lg",LDBL_DIG,num);
 			else if(num)
 				sfprintf(sh.strbuf,"%jd",(Sflong_t)num);

src/cmd/ksh93/sh/streval.c

@@ -240,7 +240,7 @@ Sfdouble_t	arith_exec(Arith_t *ep)
 				arith_error(node.value,ptr,ep->emode);
 			*++sp = num;
 			type = node.isfloat;
-			if(num > LDBL_ULLONG_MAX || num < LDBL_LLONG_MIN)
+			if(isinf(num) || isnan(num) || num > LDBL_ULLONG_MAX || num < LDBL_LLONG_MIN)
 				type = 1;
 			else
 			{
@@ -250,7 +250,8 @@ Sfdouble_t	arith_exec(Arith_t *ep)
 					type = 2;
 					d -= LDBL_LLONG_MAX;
 				}
-				if((Sflong_t)d!=d)
+				/* must use <= >= not < >, as we have reduced precision at these sizes */
+				if(d <= LDBL_LLONG_MIN || d >= LDBL_LLONG_MAX || (Sflong_t)d!=d)
 					type = 1;
 			}
 			*++tp = type;