Remove incorrect assertion in readMachODebugInfo panicking during panic

This fixes a class of bugs on macOS where a segfault happening in
a loaded dylib with no debug info would cause a panic in the panic
handler instead of simply noting that the dylib has no valid debug
info via `error.MissingDebugInfo`. An example could be code linking
some system dylib and causing some routine to segfault on say invalid
pointer value, which should normally cause Zig to print an incomplete
stack trace anchored at the currently loaded image and backtrace all
the way back to the Zig binary with valid debug info. Currently, in
a situation like this we would trigger a panic within a panic.

lib/std/debug.zig

@@ -1110,7 +1110,12 @@ fn readMachODebugInfo(allocator: mem.Allocator, macho_file: File) !ModuleDebugIn
             else => {},
         }
     }
-    assert(state == .oso_close);
+
+    switch (state) {
+        .init => return error.MissingDebugInfo,
+        .oso_close => {},
+        else => return error.InvalidDebugInfo,
+    }
 
     const symbols = try allocator.realloc(symbols_buf, symbol_index);