usdt: fail when binary doesn't exist. Fixes #1749

And add error message to hint if the problem is that the passed binary
path is not absolute or if the binary doesn't exist.

In case the PID is correct:

* but the binary couldn't be found, it will print:

  ```
  HINT: Specified binary doesn't exist.
  [...]
  ```

* but the binary is not absolute:

  ```
  HINT: Binary path should be absolute.
  [...]
  ```

Otherwise, it should keep behaving as before.

src/cc/usdt/usdt.cc

@@ -19,6 +19,7 @@
 #include <unordered_set>
 
 #include <fcntl.h>
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -393,10 +394,19 @@ extern "C" {
 void *bcc_usdt_new_frompid(int pid, const char *path) {
   USDT::Context *ctx;
 
-  if (!path)
+  if (!path) {
     ctx = new USDT::Context(pid);
-  else
+  } else {
+    struct stat buffer;
+    if (strlen(path) >= 1 && path[0] != '/') {
+      fprintf(stderr, "HINT: Binary path should be absolute.\n\n");
+      return nullptr;
+    } else if (stat(path, &buffer) == -1) {
+      fprintf(stderr, "HINT: Specified binary doesn't exist.\n\n");
+      return nullptr;
+    }
     ctx = new USDT::Context(pid, path);
+  }
   if (!ctx->loaded()) {
     delete ctx;
     return nullptr;