stage1: Fix segfault in enterexec

Prior to this commit, if `rkt enter` was executed without the `TERM`
environment variable set, a segfault could occur.
This commit treats an empty TERM environment variable as meaning 'xterm'
and avoids the segfault.

stage1/enterexec/enterexec.c

@@ -209,6 +209,12 @@ static void set_env(const char *env_file)
 static void load_env(const char *env_file, const char *keep_env_file, int entering)
 {
 	char *term = getenv("TERM"); /* useful to keep during entering. */
+	if(entering && term == NULL) {
+		// enter is meant to operate in an interactive mode, make sure we always
+		// have a TERM variable
+		pexit_if((term = strdup("xterm")) == NULL,
+			"Failed to dup default term env");
+	}
 	pexit_if(clearenv() != 0,
 		"Unable to clear environment");