Change char to auto to avoid type-limits warn on some arches (#1066)

Fixes #1059
diff --git a/src/wasm/wasm-binary.cpp b/src/wasm/wasm-binary.cpp
index 54b9a5b..3b6bbbb 100644
--- a/src/wasm/wasm-binary.cpp
+++ b/src/wasm/wasm-binary.cpp
@@ -1456,7 +1456,7 @@
   uint32_t value = 0;
   uint32_t shift = 0;
   while (1) {
-    char ch = in.get();
+    auto ch = in.get();
     if (ch == EOF)
       throw MapParseException("unexpected EOF in the middle of VLQ");
     if ((ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch < 'g')) {