Ensure NULL-termination of string when unescaping single-quotes (#514)

The for-loop is copying a string character by character and replacing `''` with `'`
by skipping a character if the current and next one are both `'`. When this
replacement doesn't occur then the target string will be filled completely
without a `\0' terminator at the end of the string.

src/bin/pgcopydb/ld_test_decoding.c

@@ -894,7 +894,7 @@ listToTuple(LogicalMessageTuple *tuple, TestDecodingColumns *cols, int count)
 			valueColumn->isQuoted = false;
 
 			int len = cur->valueLen;
-			valueColumn->val.str = (char *) calloc(len, sizeof(char));
+			valueColumn->val.str = (char *) calloc(len + 1, sizeof(char));
 
 			if (valueColumn->val.str == NULL)
 			{