i#6529: Check end of section headers mapped (#6531)

Updates the rseq_process_module() check for the section headers being in
the mapped region to check the endpoint instead of the start point.

Tested locally: now client.drwrap-test passes when it failed with the
toolchain on my local machine.

Fixes #6529

core/unix/rseq_linux.c

@@ -581,7 +581,9 @@ rseq_process_module(module_area_t *ma, bool at_map, bool saw_glibc_rseq_reg)
     ELF_SECTION_HEADER_TYPE *sec_hdr = NULL;
     char *strtab;
     ssize_t load_offs = ma->start - ma->os_data.base_address;
-    if (at_map && elf_hdr->e_shoff + ma->start < ma->end) {
+    if (at_map &&
+        elf_hdr->e_shoff + elf_hdr->e_shnum * elf_hdr->e_shentsize + ma->start <
+            ma->end) {
         sec_map = elf_hdr->e_shoff + ma->start;
         sec_hdr = (ELF_SECTION_HEADER_TYPE *)sec_map;
         /* We assume strtab is there too. */