Fix out of bounds assertion with -D_GLIBCXX_ASSERTIONS on empty string in testsuite.
--- a/lastfmlib/utils/stringoperations.cpp
+++ b/lastfmlib/utils/stringoperations.cpp
@@ -40,7 +40,7 @@ void trim(std::string& aString)
 {
     size_t begin    = 0;
     size_t end      = aString.size() - 1;
-    
+
     while ( aString[begin] == ' '  || aString[begin] == '\t'
          || aString[begin] == '\r' || aString[begin] == '\n')
     {
@@ -50,8 +50,10 @@ void trim(std::string& aString)
             break;
     }
 
-    if (begin == aString.size())
+    if (begin == aString.size()) {
         aString = "";
+        return;
+    }
 
     while ( aString[end] == ' '  || aString[end] == '\t'
          || aString[end] == '\r' || aString[end] == '\n')