Fix a panic with an invalid name section (#3509)

This commit fixes a panic which can happen on a module with an invalid
name section where one of the functions named has the index `u32::MAX`.
Previously Wasmtime would create a new `FuncIndex` with the indices
found in the name section but the sentinel `u32::MAX` causes a panic.

Cranelift otherwise limits the number of functions through `wasmparser`
which has a hard limit (lower than `u32::MAX`) so this commit applies a
fix of only recording function names for function indices that are
actually present in the module.

crates/environ/src/module_environ.rs

@@ -1277,6 +1277,11 @@ and for re-adding support for interface types you can see this issue:
                     let mut names = f.get_map()?;
                     for _ in 0..names.get_count() {
                         let Naming { index, name } = names.read()?;
+                        // Skip this naming if it's naming a function that
+                        // doesn't actually exist.
+                        if (index as usize) >= self.result.module.functions.len() {
+                            continue;
+                        }
                         let index = FuncIndex::from_u32(index);
                         self.result
                             .module
@@ -1305,6 +1310,11 @@ and for re-adding support for interface types you can see this issue:
                     let mut reader = l.get_indirect_map()?;
                     for _ in 0..reader.get_indirect_count() {
                         let f = reader.read()?;
+                        // Skip this naming if it's naming a function that
+                        // doesn't actually exist.
+                        if (f.indirect_index as usize) >= self.result.module.functions.len() {
+                            continue;
+                        }
                         let mut map = f.get_map()?;
                         for _ in 0..map.get_count() {
                             let Naming { index, name } = map.read()?;

tests/misc_testsuite/empty.wast

@@ -1,3 +1,46 @@
 (module (func (export "empty")))
 
 (invoke "empty")
+
+(module binary
+  "\00asm\01\00\00\00"    ;; module header
+
+  "\00"             ;; custom section id 0
+  "\0e"             ;; section size
+  "\04name"         ;; this is the `name` custom section
+  "\01"             ;; function name subsection
+  "\07"             ;; function name subsection size
+  "\01"             ;; 1 function name mapping
+  "\ff\ff\ff\ff\0f" ;; index == u32::MAX
+  "\00"             ;; empty string name
+)
+
+(module binary
+  "\00asm\01\00\00\00"    ;; module header
+
+  "\00"             ;; custom section id 0
+  "\10"             ;; section size
+  "\04name"         ;; this is the `name` custom section
+  "\02"             ;; local name subsection
+  "\09"             ;; local name subsection size
+  "\01"             ;; 1 indirect name map
+  "\ff\ff\ff\ff\0f" ;; index == u32::MAX (function)
+  "\01"             ;; 1 name mapping
+  "\00"             ;; index == 0 (local)
+  "\00"             ;; empty string name
+)
+
+(module binary
+  "\00asm\01\00\00\00"    ;; module header
+
+  "\00"             ;; custom section id 0
+  "\10"             ;; section size
+  "\04name"         ;; this is the `name` custom section
+  "\02"             ;; local name subsection
+  "\09"             ;; local name subsection size
+  "\01"             ;; 1 indirect name map
+  "\00"             ;; index == 0 (function)
+  "\01"             ;; 1 name mapping
+  "\ff\ff\ff\ff\0f" ;; index == u32::MAX (local)
+  "\00"             ;; empty string name
+)