Fix compare_double mishandling infinities - #1075
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The epsilon-based relative comparison in compare_double() assumes a - b and the max magnitude are both finite. For infinities that assumption breaks down: two separately allocated +Infinity numbers subtract to NaN, so the comparison reports them as unequal, while +Infinity and -Infinity subtract to +Infinity, which happens to satisfy the same comparison and reports them as equal. Both compare_double() copies (cJSON.c and the identical one in cJSON_Utils.c, used by cJSONUtils_Compare/ApplyPatch) now check for an infinite operand first and fall back to plain equality in that case, leaving the finite-number path untouched.
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Fixes #1071.
compare_double() does a relative-error comparison (fabs(a - b) <= maxVal * DBL_EPSILON) that quietly assumes both a - b and the max magnitude come out finite. That falls apart for infinities:
Both effects are visible through the public API via cJSON_Compare() on cJSON_Number items, since it calls this function directly on the two numbers' valuedouble fields.
The fix checks for an infinite operand up front and falls back to plain double equality in that case (which correctly treats two +Infinity values as equal and +Infinity/-Infinity as not equal), leaving the epsilon-based path untouched for anything finite. There's a second, identical copy of compare_double() in cJSON_Utils.c backing cJSONUtils_Compare/ApplyPatch, so I applied the same fix there and added the isnan/isinf ANSI-C fallback macros that file was missing (cJSON.c already had them for pre-C99 compilers).
Added a regression test in tests/compare_tests.c using cJSON_CreateNumber() directly, since JSON text itself has no way to spell an infinite number. Confirmed it fails against the old code and passes with the fix; full test suite (19/19) still green.