Avoid returning points from the symbol map.

Now that it uses a lookup, this is even more dangerous.  But, the maps
could be reordered while it's trying to print the pointer and cause that
data to become invalid.

This should be safe from race conditions.

Core/MIPS/MIPSDebugInterface.cpp

@@ -223,7 +223,7 @@ int MIPSDebugInterface::getColor(unsigned int address)
 	if (n==-1) return 0xFFFFFF;
 	return colors[n%6];
 }
-const char *MIPSDebugInterface::getDescription(unsigned int address) 
+std::string MIPSDebugInterface::getDescription(unsigned int address) 
 {
 	return symbolMap.GetDescription(address);
 }