Fix minor issues in user apps

Uninitialized variables, reading past array end, etc.

src/user/app/shell/shell.c

@@ -96,7 +96,12 @@ static void execute(char *command) {
   uint8_t pids_count = 0;
 
   for (uint8_t i = 0; i < subcommands_count; i++) {
-    string_trim(subcommands[i], ' ', &subcommands[i]);
+    if (!string_trim(subcommands[i], ' ', &subcommands[i])) {
+      if (subcommands_count > 1) {
+        ERR_S("Syntax error.\n");
+      }
+      break;
+    }
 
     char *argv[16];
     uint8_t argc = (uint8_t)string_split(subcommands[i], ' ', 16, argv);
@@ -166,7 +171,7 @@ exit_code_t main() {
   print_prompt();
 
   char line[256];
-  uint64_t offset;
+  uint64_t offset = 0;
 
   char chunk[64];
   uint64_t received;