Add missing string terminator

When copying the string strncpy won't add nul termination since
the string length is equal to the length specified.  Explicitly
set a nul terminator after copying to properly terminate. Found
via post-commit review.

Author: Rahila Syed <rahilasyed90@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAH2L28vt16C9xTuK+K7QZvtA3kCNWXOEiT=gEekUw3Xxp9LVQw@mail.gmail.com

src/backend/utils/mmgr/mcxt.c

@@ -1663,6 +1663,7 @@ ProcessGetMemoryContextInterrupt(void)
 			meminfo[max_stats - 1].name = dsa_allocate(MemoryStatsDsaArea, namelen + 1);
 			nameptr = dsa_get_address(MemoryStatsDsaArea, meminfo[max_stats - 1].name);
 			strncpy(nameptr, "Remaining Totals", namelen);
+			nameptr[namelen] = '\0';
 			meminfo[max_stats - 1].ident = InvalidDsaPointer;
 			meminfo[max_stats - 1].path = InvalidDsaPointer;
 			meminfo[max_stats - 1].type = 0;