[setup_payload] Fixed bug with not reducing span size properly (#18598)

Output buffer span is not reduced properly, as subspan is
passed to the base38Encode method and changed size doesn't
propagate to the outBuffer.

Added reducing outBuffer size to the subSpan + prefixLen size.

src/setup_payload/QRCodeSetupPayloadGenerator.cpp

@@ -196,6 +196,8 @@ static CHIP_ERROR payloadBase38RepresentationWithTLV(PayloadContents & payload,
         MutableCharSpan subSpan = outBuffer.SubSpan(prefixLen, outBuffer.size() - prefixLen);
         memcpy(outBuffer.data(), kQRCodePrefix, prefixLen);
         err = base38Encode(bits, subSpan);
+        // Reduce output span size to be the size of written data
+        outBuffer.reduce_size(subSpan.size() + prefixLen);
     }
 
     return err;