runtime: Fix alloc-dealloc-mismatch in buffer_single_mapped

ERROR: AddressSanitizer: alloc-dealloc-mismatch (operator new [] vs operator delete) on 0x631000014800
    #0 0x7ffbdc10f9d7 in operator delete(void*, unsigned long) (/lib64/libasan.so.6+0xad9d7)
    #1 0x7ffbd8a6646f in std::default_delete<char>::operator()(char*) const /usr/include/c++/10/bits/unique_ptr.h:85
    #2 0x7ffbd8a6646f in std::unique_ptr<char, std::default_delete<char> >::~unique_ptr() /usr/include/c++/10/bits/unique_ptr.h:361
    #3 0x7ffbd8a6646f in gr::buffer_single_mapped::~buffer_single_mapped() /home/user/src/gnuradio/gnuradio-runtime/lib/buffer_single_mapped.cc:46
    #4 0x7ffbd8c805c4 in gr::host_buffer::~host_buffer() /home/user/src/gnuradio/gnuradio-runtime/lib/host_buffer.cc:65

0x631000014800 is located 0 bytes inside of 65536-byte region [0x631000014800,0x631000024800)
allocated by thread T0 here:
    #0 0x7ffbdc10ecb7 in operator new[](unsigned long) (/lib64/libasan.so.6+0xaccb7)
    #1 0x7ffbd8c73a60 in gr::host_buffer::do_allocate_buffer(unsigned long, unsigned long) /home/user/src/gnuradio/gnuradio-runtime/lib/host_buffer.cc:119
    #2 0x7ffbd8fdaebf  (/home/user/src/gnuradio/build/gnuradio-runtime/lib/libgnuradio-runtime.so.3.10.0git+0x3475ebf)

Signed-off-by: Vasil Velichkov <vvvelichkov@gmail.com>

gnuradio-runtime/include/gnuradio/buffer_single_mapped.h

@@ -120,7 +120,7 @@ class GR_RUNTIME_API buffer_single_mapped : public buffer
 
     block_sptr d_buf_owner; // block that "owns" this buffer
 
-    std::unique_ptr<char> d_buffer;
+    std::unique_ptr<char[]> d_buffer;
 
     /*!
      * \brief constructor is private.  Use gr_make_buffer to create instances.