Fix potential panic from negative source_index in sourcemap lookups

Add missing negative value checks before @intCast operations on source_index
in Mapping.Lookup functions. While the parser validates source_index >= 0 during
parsing, defensive programming requires checking for negative values before
casting i32 to usize to prevent panics in edge cases or corrupted data.

Changes:
- displaySourceURLIfNeeded: Check source_index < 0 before array access
- getSourceCode: Add checks before two @intCast operations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/sourcemap/Mapping.zig

@@ -231,7 +231,7 @@ pub const Lookup = struct {
         // See doc comment on `external_source_names`
         if (source_map.external_source_names.len == 0)
             return null;
-        if (lookup.mapping.source_index >= source_map.external_source_names.len)
+        if (lookup.mapping.source_index < 0 or lookup.mapping.source_index >= source_map.external_source_names.len)
             return null;
 
         const name = source_map.external_source_names[@intCast(lookup.mapping.source_index)];
@@ -271,7 +271,7 @@ pub const Lookup = struct {
             // They are decompressed on demand.
             if (source_map.is_standalone_module_graph) {
                 const serialized = source_map.standaloneModuleGraphData();
-                if (index >= source_map.external_source_names.len)
+                if (index < 0 or index >= source_map.external_source_names.len)
                     return null;
 
                 const code = serialized.sourceFileContents(@intCast(index));
@@ -279,6 +279,9 @@ pub const Lookup = struct {
                 return bun.jsc.ZigString.Slice.fromUTF8NeverFree(code orelse return null);
             }
 
+            if (index < 0 or index >= source_map.external_source_names.len)
+                return null;
+
             if (provider.getSourceMap(
                 base_filename,
                 source_map.underlying_provider.load_hint,
@@ -287,9 +290,6 @@ pub const Lookup = struct {
                 if (parsed.source_contents) |contents|
                     break :bytes contents;
 
-            if (index >= source_map.external_source_names.len)
-                return null;
-
             const name = source_map.external_source_names[@intCast(index)];
 
             var buf: bun.PathBuffer = undefined;