Do not index the section list before checking the index is in range.

The section index comes straight from the ELF symbol and can be a special
index such as SHN_ABS, which is not a real section. elf.cpp binds a reference
to context.sections[section_index] before performing its own bounds check a few
lines later, and the branch taken when that check fails goes on to use the
reference anyway. Both are undefined behaviour, and a libstdc++ built with
_GLIBCXX_ASSERTIONS, which is the default for GCC on Gentoo, aborts on the
first one while recompiling the patches.

Hold a pointer instead, set only when the index is valid, and skip recording
the function address when there is no section to record it in.

--- a/lib/N64ModernRuntime/N64Recomp/src/elf.cpp
+++ b/lib/N64ModernRuntime/N64Recomp/src/elf.cpp
@@ -90,7 +90,13 @@
                 }
             }
 
-            auto& section = context.sections[section_index];
+            // section_index is taken straight from the symbol and can be a
+            // special ELF index such as SHN_ABS, which is not a real section.
+            // Binding a reference to it before the bounds check below, which
+            // is upstream's own, is undefined behaviour.
+            N64Recomp::Section* section = section_index < context.sections.size()
+                ? &context.sections[section_index]
+                : nullptr;
 
             // Check if this symbol is a function or has no type (like a regular glabel would)
             // Symbols with no type have a dummy entry created so that their symbol can be looked up for function calls
@@ -106,10 +112,10 @@
                     auto section_offset = value - elf_file.sections[section_index]->get_address();
                     uint32_t vram = static_cast<uint32_t>(value);
                     uint32_t num_instructions = type == ELFIO::STT_FUNC ? size / 4 : 0;
-                    uint32_t rom_address = static_cast<uint32_t>(section_offset + section.rom_addr);
+                    uint32_t rom_address = static_cast<uint32_t>(section_offset + section->rom_addr);
                     const uint32_t* words = reinterpret_cast<const uint32_t*>(context.rom.data() + rom_address);
 
-                    section.function_addrs.push_back(vram);
+                    section->function_addrs.push_back(vram);
                     context.functions_by_vram[vram].push_back(context.functions.size());
 
                     // Find the entrypoint by rom address in case it doesn't have vram as its value
@@ -148,7 +154,9 @@
                 } else {
                     // TODO is this case needed anymore?
                     uint32_t vram = static_cast<uint32_t>(value);
-                    section.function_addrs.push_back(vram);
+                    if (section != nullptr) {
+                        section->function_addrs.push_back(vram);
+                    }
                     context.functions_by_vram[vram].push_back(context.functions.size());
                     context.functions.emplace_back(
                         vram,