[libc] Fix GPU 'printf' on strings with padding

Summary:
We get the `strlen` to know how much memory to allocate here, but it
wasn't taking into account if the padding was larger than the string
itself. This patch sets it to an empty string so we always add the
minimum size. This implementation is slightly wasteful with memory, but
I am not concerned with a few extra bytes here and there for some memory
that gets immediately free'd.

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Differential Revision: https://phabricator.intern.facebook.com/D60251420

Author: jhuber6

libc/utils/gpu/server/rpc_server.cpp

@@ -108,6 +108,10 @@ void handle_printf(rpc::Server::Port &port) {
       if (cur_section.has_conv && cur_section.conv_name == 's' &&
           cur_section.conv_val_ptr) {
         strs_to_copy[lane].emplace_back(cur_section.conv_val_ptr);
+        // Get the minimum size of the string in the case of padding.
+        char c = '\0';
+        cur_section.conv_val_ptr = &c;
+        convert(&writer, cur_section);
       } else if (cur_section.has_conv) {
         // Ignore conversion errors for the first pass.
         convert(&writer, cur_section);