UserspaceEmulator: Handle MAP_FIXED with invalid addresses better

Old behavior: Crash due to VERIFY, unless we're completely and entirely out of
memory (m_available_ranges being empty), in which case it would return -ENOMEM.

New behavior: Return ENOMEM (and don't crash). In the case of nullptr,
also emit a helpful diagnostic.

Note that MAP_FIXED with nullptr is technically okay, but tends to be a sign
that something went wrong.

Also, this should improve mmap performance marginally, as it pulls the check out
of a loop that does not modify any parts of the check.

UE is now self-hosting! Fixes #5709.

However, this still needs some love: "ue UserspaceEmulator true" spits out tons
of error messages, probably false-positives, and takes about 229 seconds to run.
Then again, true-in-ue-in-ue-in-Qemu is three levels of emulation, so no wonder
it takes a long time! :D

Userland/DevTools/UserspaceEmulator/Emulator_syscalls.cpp

@@ -737,7 +737,14 @@ u32 Emulator::virt$mmap(u32 params_addr)
     if (params.flags & MAP_RANDOMIZED) {
         result = m_range_allocator.allocate_randomized(requested_size, params.alignment);
     } else if (params.flags & MAP_FIXED) {
-        result = m_range_allocator.allocate_specific(VirtualAddress { params.addr }, requested_size);
+        if (params.addr)
+            result = m_range_allocator.allocate_specific(VirtualAddress { params.addr }, requested_size);
+        else {
+            // mmap(nullptr, …, MAP_FIXED) is technically okay, but tends to be a bug.
+            // Therefore, refuse to be helpful.
+            reportln("\n=={}==  \033[31;1mTried to mmap at nullptr with MAP_FIXED.\033[0m, 0x{:x} bytes.", params.size);
+            dump_backtrace();
+        }
     } else {
         result = m_range_allocator.allocate_anywhere(requested_size, params.alignment);
     }

Userland/DevTools/UserspaceEmulator/RangeAllocator.cpp

@@ -149,9 +149,12 @@ Optional<Range> RangeAllocator::allocate_specific(VirtualAddress base, size_t si
     VERIFY((size % PAGE_SIZE) == 0);
 
     Range allocated_range(base, size);
+    if (!m_total_range.contains(allocated_range)) {
+        dbgln("Unallocatable mmap request?! {:p}+{:p}", base.get(), size);
+        return {};
+    }
     for (size_t i = 0; i < m_available_ranges.size(); ++i) {
         auto& available_range = m_available_ranges[i];
-        VERIFY(m_total_range.contains(allocated_range));
         if (!available_range.contains(base, size))
             continue;
         if (available_range == allocated_range) {