Fix 'zdb -o' segmentation fault

Fix a regression accidentally introduced by OpenZFS 7280 in ed828c0: since
whether to accept NULL as a valid first parameter in strchr() is implementation
specific we add an additional check to avoid crashing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #5917

lib/libzpool/util.c

@@ -182,7 +182,7 @@ set_global_var(char *arg)
 	    "little-endian systems\n");
 	return (ENOTSUP);
 #endif
-	if ((varval = strchr(arg, '=')) != NULL) {
+	if (arg != NULL && (varval = strchr(arg, '=')) != NULL) {
 		*varval = '\0';
 		varval++;
 		val = strtoull(varval, NULL, 0);