Bug 1935437 - Check in case libName could be nullptr before passing i…

…t inside the std::string constructor in LoadedLibraryInfo constructor r=profiler-reviewers,florian

Before the shared-libraries code deduplication, we were using nsCString for the
`LoadedLibraryInfo::mName`. After the deduplication we started to use
`std::string`.

Before, it was fine to pass nullptr as a parameter to nsCString as it could
easily handle it and act as if it's an empty string. But it's an undefined
behavior on `std::string`. And passing a nullptr actively crashes Firefox when
there is a BHR hang detected (as it uses the shared libraries code).

So now, instead of directly passing `dl_info->dlpi_name` inside `std::string`,
I'm checking if it's a nullptr or not, and if so I'm passing an empty string
so we don't crash there.

Differential Revision: https://phabricator.services.mozilla.com/D231926

UltraBlame original commit: 2cb187c7d3f15302a7c2665b10157f50155cdbf1

mozglue/baseprofiler/core/shared-libraries-linux.cc

@@ -3607,17 +3607,32 @@ end
 ;
 }
 }
+const
+char
+*
+libName
+=
+dl_info
+-
+>
+dlpi_name
+?
+dl_info
+-
+>
+dlpi_name
+:
+"
+"
+;
 libInfoList
 -
 >
 push_back
 (
 LoadedLibraryInfo
 (
-dl_info
--
->
-dlpi_name
+libName
 baseAddress
 firstMappingStart
 lastMappingEnd