!68 fix exec crash

Merge pull request !68 from 张吉东/fix_exec_release_503

BUILD.gn

@@ -244,6 +244,9 @@ if (defined(ohos_lite)) {
       "-DMKSH_UNLIMITED",
       "-DMKSH_BUILD_R=593",
     ]
+
+    defines = [ "MKSH_OH_ADAPT" ]
+
     ldflags = [
       "-pie",
       "-Wl,-z,relro",

exec.c

@@ -1167,6 +1167,12 @@ builtin(const char *name, int (*func) (const char **))
 struct tbl *
 findcom(const char *name, int flags)
 {
+#ifdef MKSH_OH_ADAPT
+	// fix crash for 'exec -a0'
+	if (name == NULL) {
+		return NULL;
+	}
+#endif
 	static struct tbl temp;
 	uint32_t h = hash(name);
 	struct tbl *tp = NULL, *tbi;