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Fix undefined behavior in `readlong`
See the new comment for what caused the UB, and how it was fixed

--- a/src/link/object.c

+++ b/src/link/object.c

@@ -55,7 +55,14 @@

 		if (byte == EOF)

 			return INT64_MAX;

-		value |= (uint8_t)byte << shift;

+		/* This must be casted to `unsigned`, not `uint8_t`. Rationale:

+		 * the type of the shift is the type of `byte` after undergoing

+		 * integer promotion, which would be `int` if this was casted to

+		 * `uint8_t`, because int is large enough to hold a byte. This

+		 * however causes values larger than 127 to be too large when

+		 * shifted, potentially triggering undefined behavior.

+		 */

+		value |= (unsigned)byte << shift;

 	}

 	return value;

 }