[gdb] Check strpbrk against nullptr

In noticed two occurrences of "if (strpbrk (...))".

Fix this style issue by checking against nullptr.

gdb/go32-nat.c

@@ -697,7 +697,7 @@ go32_nat_target::create_inferior (const char *exec_file,
 		      "not enough memory.\n"));
 
   /* Parse the command line and create redirections.  */
-  if (strpbrk (args, "<>"))
+  if (strpbrk (args, "<>") != nullptr)
     {
       if (redir_cmdline_parse (args, &child_cmd) == 0)
 	args = child_cmd.command;

gdbsupport/common-inferior.cc

@@ -59,7 +59,7 @@ escape_characters (const char *arg, const char *special)
 #ifdef __MINGW32__
       bool quoted = false;
 
-      if (strpbrk (arg, special))
+      if (strpbrk (arg, special) != nullptr)
 	{
 	  quoted = true;
 	  result += quote;