itest: allow map_physmem to return 0 in limited cases

On some systems, RAM starts at address 0. If the user executes itest
against address 0 on such a system, it will call map_physmem(0, ...)
which will return 0 back; mapping only changes the address on sandbox.
This causes itest to believe map_physmem() has failed, and hence fails
the comparison.

Fix itest so that it allows map_physmem() to return 0 /if/ the orignal
address passed to it was also 0.

This fixes "tegra-uboot-flasher flash" on Tegra20.

This has the disadvantage that on sandbox, failed mapping attempts for
address 0 are not detected. Instead, should the code only call
map_physmem() on sandbox? Or, should map_physmem() return its error status
some other way. Or, should the special case only be allowed on systems
where the base of RAM is 0 somehow?

Fixes: 7861204c9af7 ("itest: make memory access work under sandbox")
Signed-off-by: Stephen Warren <swarren@nvidia.com>

cmd/itest.c

@@ -59,7 +59,7 @@ static long evalexp(char *s, int w)
 	if (s[0] == '*') {
 		addr = simple_strtoul(&s[1], NULL, 16);
 		buf = map_physmem(addr, w, MAP_WRBACK);
-		if (!buf) {
+		if (!buf && addr) {
 			puts("Failed to map physical memory\n");
 			return 0;
 		}