Tools: Test: Fix for dBFS level measure for muted audio

The log() function returns -Inf for level measure for stream of
digital zeros. It is correct but requires extra handling in the
upper level functions to avoid unnecessary tests fails. Adding of
offset 1e-20 results to -196.99 dBFS be the smallest possible
reported value. The function remains sufficient for all audio
measurements with 16 - 32 bit formats.

The license text is also updated to current version.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

tools/test/audio/std_utils/level_dbfs.m

@@ -9,38 +9,12 @@ function dbfs = level_dbfs(x)
 % dbfs - sigmal level in dBFS
 %
 
-%%
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 % Author: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>
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 %% Reference AES 17 3.12.3
 level_ms = mean(x.^2);
-dbfs = 10*log10(level_ms) + 20*log10(sqrt(2));
+dbfs = 10*log10(level_ms + 1e-20) + 20*log10(sqrt(2));
 
 end