Change getc return type from char to int32_t

The return type of `getc(3)` is a signed integer,
with negative values indicating an error. The changed
line stored the return value in an *unsigned* integer,
then tested it against a negative error constant, so
the error condition would never be detected. This
also results in a warning on GCC 10.

Further information:
- [`getc(3)`](https://man7.org/linux/man-pages/man3/fgetc.3.html)
- [C FAQ question 12.1](http://c-faq.com/stdio/getcharc.html)

software/writeentropy.c

@@ -25,7 +25,7 @@ static uint32_t readNumberFromFile(char *fileName) {
         exit(1);
     }
     uint32_t value = 0u;
-    char c;
+    int32_t c;
     while( (c = getc(file)) != EOF
            && '0' <= c && c <= '9' ) {
         value *= 10;