Fix dehumanize_number() to correctly detect overflows

Do not allow numbers greated than INT64_MAX and smaller than INT64_MIN.
Clarify the positive sign value by prefixing it with an explicit +.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=66909

test/humanize.c

@@ -25,6 +25,7 @@
  */
 
 #include <assert.h>
+#include <errno.h>
 #include <stdlib.h>
 
 int
@@ -60,5 +61,17 @@ main(int argc, char **argv)
 	assert(dehumanize_number("-3G", &val) == 0);
 	assert(val == -3221225472LL);
 
+	assert(dehumanize_number("9223372036854775807", &val) == 0);
+	assert(val == INT64_MAX);
+
+	assert(dehumanize_number("9223372036854775808", &val) == -1);
+	assert(errno == ERANGE);
+
+	assert(dehumanize_number("-9223372036854775808", &val) == 0);
+	assert(val == INT64_MIN);
+
+	assert(dehumanize_number("-9223372036854775809", &val) == -1);
+	assert(errno == ERANGE);
+
 	return 0;
 }