fuzz: Limit memories in the instantiate-many fuzzer (#3902)

A fuzz bug was hit last night where the root of the fuzz bug appears to
be exhaustion of the virtual address space. The specific case in
question instantiated a module with ~100 memories ~100 times, and each
memory reserved ~8gb of the virtual address space. This takes around 47
bits of addressable memory which is mighty close to the limit of what
can be done on x86_64, so this commit reduces the number of memories
that an instance may have when coming out of `wasm-smith`.

fuzz/fuzz_targets/instantiate-many.rs

@@ -24,6 +24,16 @@ fn run(data: &[u8]) -> Result<()> {
     // use timeouts or ensure that the generated wasm code will terminate.
     config.module_config.config.allow_start_export = false;
 
+    // Wasm linear memories take roughly ~8gb of virtual address space. Down
+    // below we could instantiate up to 300 modules. Conservatively estimating
+    // that we have 46 bits of address space to work with (technically 48 on
+    // x86_64, but take some out for kernel stuff and some for asan stuff) that
+    // gives us a budget of ~27 memories per instance. Reduce that a bit further
+    // and make sure that no instance has more than 10 linear memories to ensure
+    // that even if the maximum were created it should still fit in the linear
+    // address space.
+    config.module_config.config.max_memories = config.module_config.config.max_memories.min(10);
+
     // Create the modules to instantiate
     let modules = (0..u.int_in_range(1..=MAX_MODULES)?)
         .map(|_| Ok(config.generate(&mut u, None)?.to_bytes()))