While trying to concatenate two po files, I got:
msgcat: memory exhausted

Debugging in gdb, I realized that one of the failing po files has a non
standard header that contains extra empty line:
msgid ""
msgstr ""
"Project-Id-Version: @PACKAGE@\n"
"Report-Msgid-Bugs-To: \n"
"POT-Creation-Date: 2021-09-16 17:13+0200\n"
"PO-Revision-Date: 2005-07-29 15:37+0530\n"
"Last-Translator: Priyavert Sharma<priyavert.sharma@agreeya.com>\n"
"Language-Team: AgreeYa Solutions <linux_team@agreeya.com>\n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Plural-Forms: nplurals=2; plural=(n!=1);\n"
"\n"

The bug is triggered in msgl-header.c: message_list_header_list()

       while (*h != '\0')
          {
            char *enh = strchr (h, ':');
            enh++;
            char * msgid = (char *)XNMALLOC (((enh - h) + 1), char);

If the line does not contain ":", then enh is zero, and (enh - h) + 1 is
evaluated to an absurd number.

This is not an upstream bug. The smart header merge feature is introduced by:
0002-msgcat-Merge-headers-when-use-first.patch