trivial: shell: return 0 if fromdec/hex is passd null

This was actually working anyways, because NULL pointers don't trap,
and the pointed-to char is currently 0x98, not a digit.

But I prefer to fix it before we do one of the following:
  - move to a new cpu that traps on null
  - use RAM at a different address than zeero
  - use "mvi r0, 0" instead of "xor r0,r0,r0" (would parse as 4).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

shell/shell.c

@@ -216,7 +216,7 @@ const char *fromhex(const char *hex, int *v)
 {
 	int o = 0;
 
-	for (; *hex; ++hex) {
+	for (; hex && *hex; ++hex) {
 		if (*hex >= '0' && *hex <= '9') {
 			o = (o << 4) + (*hex - '0');
 		} else if (*hex >= 'A' && *hex <= 'F') {
@@ -236,7 +236,7 @@ const char *fromdec(const char *dec, int *v)
 {
 	int o = 0;
 
-	for (; *dec; ++dec) {
+	for (; dec && *dec; ++dec) {
 		if (*dec >= '0' && *dec <= '9') {
 			o = (o * 10) + (*dec - '0');
 		} else {