reject overlong base64 vlq in source map parser

[fr-manvi]

An overlong Base64 VLQ shifts a 32-bit accumulator by 32 or more bits in ReadBase64VLQ:

• the loop grows shift by 5 for every continuation digit with no cap, unlike the LEB128 readers in webassembly.cc and dwarf/dwarf_util.cc which stop at a max shift
• a mappings string with 8+ continuation digits reaches shift 35, so digit << shift on the uint32 is an out-of-range left shift (UBSan: shift exponent 35 is too large for 32-bit type)
• reachable from any source map bloaty loads via --source-map, so a crafted .map triggers it under the undefined-sanitizer fuzz config

Reject once shift hits 32; a valid 32-bit VLQ never gets there, so valid maps are unaffected. Added a regression test under tests/wasm.