COMMON: Fix assingning a String to a U32String

When assigning a String or char * to an existing U32String, the code
assumed that the current _str pointed to the internal storage. But
that is not the case if the U32String contains a string longer
than the internal storage capacity prior to the assignment, and
this led to various memory issues (and usually a crash down the
line).

common/ustr.cpp

@@ -127,6 +127,7 @@ U32String &U32String::operator=(const U32String &str) {
 }
 
 U32String &U32String::operator=(const String &str) {
+	clear();
 	initWithCStr(str.c_str(), str.size());
 	return *this;
 }
@@ -136,6 +137,7 @@ U32String &U32String::operator=(const value_type *str) {
 }
 
 U32String &U32String::operator=(const char *str) {
+	clear();
 	initWithCStr(str, strlen(str));
 	return *this;
 }