Print an error message and return instead of crash

When the address of a symbol couldn't be found, the app should return
instead of crash. The address is validated by the caller, which returns
false if address is NULL. Eventually the loader gets the error message,
and reverts back to the previous good image.

b/357947368

Change-Id: I74b9aa600c281a74ef817af0cd97f0cda4ab91cd

starboard/elf_loader/exported_symbols.cc

@@ -632,7 +632,9 @@ const void* ExportedSymbols::Lookup(const char* name) {
   const void* address = map_[name];
   // Any symbol that is not registered as part of the Starboard API in the
   // constructor of this class is a leak, and is an error.
-  SB_CHECK(address) << "Failed to retrieve the address of '" << name << "'.";
+  if (!address) {
+    SB_LOG(ERROR) << "Failed to retrieve the address of '" << name << "'.";
+  }
   return address;
 }
 

starboard/elf_loader/exported_symbols.h

@@ -29,6 +29,7 @@ namespace elf_loader {
 class ExportedSymbols {
  public:
   ExportedSymbols();
+  // Returns the address of the symbol |name|. If it's not found, returns NULL.
   const void* Lookup(const char* name);
 
  private: