Integer overflows in readelf get_data

Integer overflows in readelf get_data

I noticed the test for overflow of amt = size * nmemb in get_data
wasn't effective.  An obvious example of nmemb = 3 and size = half max
value overflows but doesn't result in amt < nmemb.  This patch fixes
this problem and reports a size truncation or overflow rather than out
of memory in more cases.

	* readelf.c (get_data): Improve overflow checks.