memory analyzer: fail when gdb cannot be started

When gdb wasn't available, the forked child would not terminate and the
parent would wait forever.

Author: tautschnig

src/memory-analyzer/gdb_api.cpp

@@ -133,12 +133,11 @@ void gdb_apit::create_gdb_process()
     // Only reachable, if execvp failed
     int errno_value = errno;
     dprintf(pipe_output[1], "errno in child: %s\n", strerror(errno_value));
+    throw gdb_interaction_exceptiont("could not run gdb");
   }
   else
   {
     // parent process
-    gdb_state = gdb_statet::CREATED;
-
     close(pipe_input[0]);
     close(pipe_output[1]);
 
@@ -148,6 +147,11 @@ void gdb_apit::create_gdb_process()
     // get stream for writing to gdb
     command_stream = fdopen(pipe_input[1], "w");
 
+    if(waitpid(gdb_process, 0, WNOHANG) == 0)
+      throw gdb_interaction_exceptiont("could not run gdb");
+    else
+      gdb_state = gdb_statet::CREATED;
+
     std::string line = read_most_recent_line();
     CHECK_RETURN(
       has_prefix(line, R"(~"done)") ||