[Backport branch-8-6] Fix WCS 2.0 GetCoverage DoS from non-positive RESOLUTION (GHSA-6jr5-rc9c-p3cj) - #7589

…c9c-p3cj)

An unauthenticated, remotely reachable denial-of-service vulnerability
exists in MapServer's WCS 2.0 GetCoverage path. The parser accepts any
RESOLUTION value that parses as a double — including negative, zero, and
nan — without a > 0 / finiteness check. The resulting negative image
dimensions are then cast to size_t (huge value) inside
msSmallMalloc()/msSmallCalloc(). Allocation fails, and MapServer's
"safe" allocator terminates the process via exit(1). A single crafted
request kills the handling CGI/FastCGI worker.

Fix:
1. msWCSParseResolutionString20(): After parsing, reject non-finite
   values and values <= 0 with InvalidParameterValue. This mirrors the
   existing validation in msWCSParseScaleString20() which already
   rejects scale <= 0.
2. msWCSGetCoverage20(): Add a defensive guard that rejects width <= 0
   or height <= 0 before the existing MAXSIZE check. This catches any
   edge case where a non-positive dimension reaches this point.

Tests added in msautotest/wxs/wcs_simple.map:
- Negative RESOLUTION → InvalidParameterValue (was exit(1))
- Zero RESOLUTION → InvalidParameterValue (was raw alloc error)